dockerfile-maven-plugin極簡教程

Naylor發表於2020-10-12

一、簡介

maven是一個專案依賴管理和構建的工具,dockerfile-maven-plugin是一個maven的外掛,主要作用是在專案構建的時候生成基於專案的docker映象檔案。

簡而言之,此外掛將maven和docker進行整合。

正常情況下,我們在開發了一個應用程式後,會使用maven進行打包,生成對應的jar檔案。而後,會使用docker將jar檔案build成一個映象(docker image)。之後,就可以在docker daemon中建立基於映象的容器,並可提供服務了。

dockerfile-maven-plugin的目標就是將maven的打包過程和docker的build過程結合在一起,當成功打包,既生成了對應的jar,也已生成了對應的docker映象。當然,這只是最基礎的功能,更詳細的功能參見:https://github.com/spotify/dockerfile-maven

二、概述

我們知道maven是apache公司開發的一個產品,但是dockerfile-maven-plugin並不是apache官方開發的外掛,是由一個叫做Spotify的組織開發的。

github主頁:https://spotify.github.io/

github開源地址:https://github.com/spotify/dockerfile-maven

本文僅討論如何基於一個Spring Boot的專案生成對應的docker映象。

基本的原理如下:

  • 首先,dockerfile-maven-plugin外掛已經儲存在maven的倉庫中
  • 然後,當在本地開發的時候,需要在專案的pom檔案中引入此外掛,在pom-build-plugins下面增加plugin配置節點
  • 再然後,在executions節點中配置此外掛如何工作;並且在configuration節點中加入需要的配置資訊
  • 最後,當我們執行mvn package的時候就可以得到docker image 了

環境:

  • Ideal版本:2020.01
  • java版本:8
  • maven版本:3.6.1
  • docker版本:19.03.12

ideal和docker deamon執行在同一臺機器上面

三、將spring-boot-app打包成docker映象

建立示例應用

使用ideal自帶的Spring Initializr生成一個Spring Web 的示例專案

app對外提供一個hello的介面,訪問該介面可以得到Hello,World的響應結果。應用主啟動類程式碼如下:

package com.naylor.dockerfilemavenplugin;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/")
@SpringBootApplication
public class DockerfileMavenPluginApplication {

    public static void main(String[] args) {
        SpringApplication.run(DockerfileMavenPluginApplication.class, args);
    }

    @GetMapping("/hello")
    public  String hello(){
        return  "Hello,World";
    }

}


編譯並執行專案,在瀏覽器中訪問「http://127.0.0.1:8080/hello」 可以得到預期的響應結果

修改pom檔案

在pom中增加對dockerfile-maven-plugin外掛的引用,核心程式碼如下:

 <!--   dockerfile-maven-plugin      -->
            <plugin>
                <groupId>com.spotify</groupId>
                <artifactId>dockerfile-maven-plugin</artifactId>
                <version>1.3.6</version>
                <executions>
                    <execution>
                        <id>default</id>
                        <goals>
                            <goal>build</goal>
                            
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <repository>com.naylor/${project.artifactId}</repository>
                    <tag>${project.version}</tag>
                    <buildArgs>
                        <JAR_FILE>${project.build.finalName}.jar</JAR_FILE>
                    </buildArgs>
                </configuration>
            </plugin>

其中:

  • g,a,v 為對外掛的引用

  • executions中的build標識,在maven的packege環節執行此外掛

  • configuration中的repository是生成的映象的repository資訊

  • tag為映象的tag資訊

  • buildArgs是在docker構建映象過程中的引數,此處定義的JAR_FILE引數在執行docker build 的時候會消費

完整的pom檔案如下:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.4.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.naylor</groupId>
    <artifactId>dockerfile-maven-plugin</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>dockerfile-maven-plugin</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>

            <!--   dockerfile-maven-plugin      -->
            <plugin>
                <groupId>com.spotify</groupId>
                <artifactId>dockerfile-maven-plugin</artifactId>
                <version>1.3.6</version>
                <executions>
                    <execution>
                        <id>default</id>
                        <goals>
                            <goal>build</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <repository>com.naylor/${project.artifactId}</repository>
                    <tag>${project.version}</tag>
                    <buildArgs>
                        <JAR_FILE>${project.build.finalName}.jar</JAR_FILE>
                    </buildArgs>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>


增加Dockerfile檔案

在專案根目錄(和pom檔案在同一級)新建一個Dokerfile檔案,檔案內容如下:

FROM java:8
EXPOSE 8080
ARG JAR_FILE
ADD target/${JAR_FILE} /app.jar
ENTRYPOINT ["java", "-jar","/app.jar"]

使用Maven打包應用

首先清理一下maven工程,在ideal的Maven面版中點選Lifecycle-clean或者使用命令列執行mvn clean。

然後,使用maven構建app,在ideal的Maven面版中點選Liftcycle-package或者使用命令列執行 mvn package

再然後在命令列工具中執行docker image ls ,如果不出意外,可以看到一個repository為com.naylor/dockerfile-maven-plugin的docker映象。

執行應用映象

在命令列工具中執行如下命令執行容器:

docker run -d -p 8081:8080 ImageId
  • ImageId為上一步生成的映象的id,每次生成的映象id都不一樣
  • 此命令作用為基於ImageId構建一個容器,將宿主機的8081埠對映到容器的8080埠

在宿主機瀏覽器中訪問「http://127.0.0.1:8081/hello」可以得到Hello,World的響應。

四、分析mvn package 命令的控制檯輸出

通過mvn package的控制檯輸出,我們可以清晰的觀察到整個流程的執行步驟,完整的輸出如下:

/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/bin/java -Dmaven.multiModuleProjectDirectory=/Users/chenhd/code/DebrisApp_Springboot/debris-app "-Dmaven.home=/Applications/IntelliJ IDEA 2.app/Contents/plugins/maven/lib/maven3" "-Dclassworlds.conf=/Applications/IntelliJ IDEA 2.app/Contents/plugins/maven/lib/maven3/bin/m2.conf" "-Dmaven.ext.class.path=/Applications/IntelliJ IDEA 2.app/Contents/plugins/maven/lib/maven-event-listener.jar" "-javaagent:/Applications/IntelliJ IDEA 2.app/Contents/lib/idea_rt.jar=58649:/Applications/IntelliJ IDEA 2.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath "/Applications/IntelliJ IDEA 2.app/Contents/plugins/maven/lib/maven3/boot/plexus-classworlds-2.6.0.jar" org.codehaus.classworlds.Launcher -Didea.version2020.1 package
[INFO] Scanning for projects...
[INFO] 
[INFO] -----------------< com.naylor:dockerfile-maven-plugin >-----------------
[INFO] Building dockerfile-maven-plugin 0.0.1-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- maven-resources-plugin:3.1.0:resources (default-resources) @ dockerfile-maven-plugin ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] Copying 1 resource
[INFO] Copying 0 resource
[INFO] 
[INFO] --- maven-compiler-plugin:3.8.1:compile (default-compile) @ dockerfile-maven-plugin ---
[INFO] Nothing to compile - all classes are up to date
[INFO] 
[INFO] --- maven-resources-plugin:3.1.0:testResources (default-testResources) @ dockerfile-maven-plugin ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory /Users/chenhd/code/DebrisApp_Springboot/debris-app/dockerfile-maven-plugin/src/test/resources
[INFO] 
[INFO] --- maven-compiler-plugin:3.8.1:testCompile (default-testCompile) @ dockerfile-maven-plugin ---
[INFO] Nothing to compile - all classes are up to date
[INFO] 
[INFO] --- maven-surefire-plugin:2.22.2:test (default-test) @ dockerfile-maven-plugin ---
[INFO] 
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests
13:49:50.713 [main] DEBUG org.springframework.test.context.BootstrapUtils - Instantiating CacheAwareContextLoaderDelegate from class [org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate]
13:49:50.735 [main] DEBUG org.springframework.test.context.BootstrapUtils - Instantiating BootstrapContext using constructor [public org.springframework.test.context.support.DefaultBootstrapContext(java.lang.Class,org.springframework.test.context.CacheAwareContextLoaderDelegate)]
13:49:50.762 [main] DEBUG org.springframework.test.context.BootstrapUtils - Instantiating TestContextBootstrapper for test class [com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests] from class [org.springframework.boot.test.context.SpringBootTestContextBootstrapper]
13:49:50.781 [main] INFO org.springframework.boot.test.context.SpringBootTestContextBootstrapper - Neither @ContextConfiguration nor @ContextHierarchy found for test class [com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests], using SpringBootContextLoader
13:49:50.785 [main] DEBUG org.springframework.test.context.support.AbstractContextLoader - Did not detect default resource location for test class [com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests]: class path resource [com/naylor/dockerfilemavenplugin/DockerfileMavenPluginApplicationTests-context.xml] does not exist
13:49:50.785 [main] DEBUG org.springframework.test.context.support.AbstractContextLoader - Did not detect default resource location for test class [com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests]: class path resource [com/naylor/dockerfilemavenplugin/DockerfileMavenPluginApplicationTestsContext.groovy] does not exist
13:49:50.785 [main] INFO org.springframework.test.context.support.AbstractContextLoader - Could not detect default resource locations for test class [com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests]: no resource found for suffixes {-context.xml, Context.groovy}.
13:49:50.786 [main] INFO org.springframework.test.context.support.AnnotationConfigContextLoaderUtils - Could not detect default configuration classes for test class [com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests]: DockerfileMavenPluginApplicationTests does not declare any static, non-private, non-final, nested classes annotated with @Configuration.
13:49:50.826 [main] DEBUG org.springframework.test.context.support.ActiveProfilesUtils - Could not find an 'annotation declaring class' for annotation type [org.springframework.test.context.ActiveProfiles] and class [com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests]
13:49:50.926 [main] DEBUG org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider - Identified candidate component class: file [/Users/chenhd/code/DebrisApp_Springboot/debris-app/dockerfile-maven-plugin/target/classes/com/naylor/dockerfilemavenplugin/DockerfileMavenPluginApplication.class]
13:49:50.932 [main] INFO org.springframework.boot.test.context.SpringBootTestContextBootstrapper - Found @SpringBootConfiguration com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplication for test class com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests
13:49:51.069 [main] DEBUG org.springframework.boot.test.context.SpringBootTestContextBootstrapper - @TestExecutionListeners is not present for class [com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests]: using defaults.
13:49:51.070 [main] INFO org.springframework.boot.test.context.SpringBootTestContextBootstrapper - Loaded default TestExecutionListener class names from location [META-INF/spring.factories]: [org.springframework.boot.test.mock.mockito.MockitoTestExecutionListener, org.springframework.boot.test.mock.mockito.ResetMocksTestExecutionListener, org.springframework.boot.test.autoconfigure.restdocs.RestDocsTestExecutionListener, org.springframework.boot.test.autoconfigure.web.client.MockRestServiceServerResetTestExecutionListener, org.springframework.boot.test.autoconfigure.web.servlet.MockMvcPrintOnlyOnFailureTestExecutionListener, org.springframework.boot.test.autoconfigure.web.servlet.WebDriverTestExecutionListener, org.springframework.boot.test.autoconfigure.webservices.client.MockWebServiceServerTestExecutionListener, org.springframework.test.context.web.ServletTestExecutionListener, org.springframework.test.context.support.DirtiesContextBeforeModesTestExecutionListener, org.springframework.test.context.support.DependencyInjectionTestExecutionListener, org.springframework.test.context.support.DirtiesContextTestExecutionListener, org.springframework.test.context.transaction.TransactionalTestExecutionListener, org.springframework.test.context.jdbc.SqlScriptsTestExecutionListener, org.springframework.test.context.event.EventPublishingTestExecutionListener]
13:49:51.096 [main] DEBUG org.springframework.boot.test.context.SpringBootTestContextBootstrapper - Skipping candidate TestExecutionListener [org.springframework.test.context.transaction.TransactionalTestExecutionListener] due to a missing dependency. Specify custom listener classes or make the default listener classes and their required dependencies available. Offending class: [org/springframework/transaction/interceptor/TransactionAttributeSource]
13:49:51.098 [main] DEBUG org.springframework.boot.test.context.SpringBootTestContextBootstrapper - Skipping candidate TestExecutionListener [org.springframework.test.context.jdbc.SqlScriptsTestExecutionListener] due to a missing dependency. Specify custom listener classes or make the default listener classes and their required dependencies available. Offending class: [org/springframework/transaction/interceptor/TransactionAttribute]
13:49:51.098 [main] INFO org.springframework.boot.test.context.SpringBootTestContextBootstrapper - Using TestExecutionListeners: [org.springframework.test.context.web.ServletTestExecutionListener@5acf93bb, org.springframework.test.context.support.DirtiesContextBeforeModesTestExecutionListener@7e7be63f, org.springframework.boot.test.mock.mockito.MockitoTestExecutionListener@6cd28fa7, org.springframework.boot.test.autoconfigure.SpringBootDependencyInjectionTestExecutionListener@614ca7df, org.springframework.test.context.support.DirtiesContextTestExecutionListener@4738a206, org.springframework.test.context.event.EventPublishingTestExecutionListener@66d3eec0, org.springframework.boot.test.mock.mockito.ResetMocksTestExecutionListener@1e04fa0a, org.springframework.boot.test.autoconfigure.restdocs.RestDocsTestExecutionListener@1af2d44a, org.springframework.boot.test.autoconfigure.web.client.MockRestServiceServerResetTestExecutionListener@18d87d80, org.springframework.boot.test.autoconfigure.web.servlet.MockMvcPrintOnlyOnFailureTestExecutionListener@618425b5, org.springframework.boot.test.autoconfigure.web.servlet.WebDriverTestExecutionListener@58695725, org.springframework.boot.test.autoconfigure.webservices.client.MockWebServiceServerTestExecutionListener@543588e6]
13:49:51.114 [main] DEBUG org.springframework.test.context.support.AbstractDirtiesContextTestExecutionListener - Before test class: context [DefaultTestContext@209da20d testClass = DockerfileMavenPluginApplicationTests, testInstance = [null], testMethod = [null], testException = [null], mergedContextConfiguration = [WebMergedContextConfiguration@e15b7e8 testClass = DockerfileMavenPluginApplicationTests, locations = '{}', classes = '{class com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplication}', contextInitializerClasses = '[]', activeProfiles = '{}', propertySourceLocations = '{}', propertySourceProperties = '{org.springframework.boot.test.context.SpringBootTestContextBootstrapper=true}', contextCustomizers = set[org.springframework.boot.test.context.filter.ExcludeFilterContextCustomizer@27ae2fd0, org.springframework.boot.test.json.DuplicateJsonObjectContextCustomizerFactory$DuplicateJsonObjectContextCustomizer@4278a03f, org.springframework.boot.test.mock.mockito.MockitoContextCustomizer@0, org.springframework.boot.test.web.client.TestRestTemplateContextCustomizer@2bbf180e, org.springframework.boot.test.autoconfigure.properties.PropertyMappingContextCustomizer@0, org.springframework.boot.test.autoconfigure.web.servlet.WebDriverContextCustomizerFactory$Customizer@96def03, org.springframework.boot.test.context.SpringBootTestArgs@1], resourceBasePath = 'src/main/webapp', contextLoader = 'org.springframework.boot.test.context.SpringBootContextLoader', parent = [null]], attributes = map['org.springframework.test.context.web.ServletTestExecutionListener.activateListener' -> true]], class annotated with @DirtiesContext [false] with mode [null].
13:49:51.152 [main] DEBUG org.springframework.test.context.support.TestPropertySourceUtils - Adding inlined properties to environment: {spring.jmx.enabled=false, org.springframework.boot.test.context.SpringBootTestContextBootstrapper=true}

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.3.4.RELEASE)

2020-10-12 13:49:51.449  INFO 16693 --- [           main] .d.DockerfileMavenPluginApplicationTests : Starting DockerfileMavenPluginApplicationTests on neiyo with PID 16693 (started by chenhd in /Users/chenhd/code/DebrisApp_Springboot/debris-app/dockerfile-maven-plugin)
2020-10-12 13:49:51.451  INFO 16693 --- [           main] .d.DockerfileMavenPluginApplicationTests : No active profile set, falling back to default profiles: default
2020-10-12 13:49:52.458  INFO 16693 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2020-10-12 13:49:52.732  INFO 16693 --- [           main] .d.DockerfileMavenPluginApplicationTests : Started DockerfileMavenPluginApplicationTests in 1.559 seconds (JVM running for 2.86)
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 2.313 s - in com.naylor.dockerfilemavenplugin.DockerfileMavenPluginApplicationTests
2020-10-12 13:49:53.042  INFO 16693 --- [extShutdownHook] o.s.s.concurrent.ThreadPoolTaskExecutor  : Shutting down ExecutorService 'applicationTaskExecutor'
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] 
[INFO] --- maven-jar-plugin:3.2.0:jar (default-jar) @ dockerfile-maven-plugin ---
[INFO] Building jar: /Users/chenhd/code/DebrisApp_Springboot/debris-app/dockerfile-maven-plugin/target/dockerfile-maven-plugin-0.0.1-SNAPSHOT.jar
[INFO] 
[INFO] --- spring-boot-maven-plugin:2.3.4.RELEASE:repackage (repackage) @ dockerfile-maven-plugin ---
[INFO] Replacing main artifact with repackaged archive
[INFO] 
[INFO] --- dockerfile-maven-plugin:1.3.6:build (default) @ dockerfile-maven-plugin ---
[INFO] Building Docker context /Users/chenhd/code/DebrisApp_Springboot/debris-app/dockerfile-maven-plugin
[INFO] 
[INFO] Image will be built as com.naylor/dockerfile-maven-plugin:0.0.1-SNAPSHOT
[INFO] 
[INFO] Step 1/5 : FROM java:8
[INFO] 
[INFO] Pulling from library/java
[INFO] Digest: sha256:c1ff613e8ba25833d2e1940da0940c3824f03f802c449f3d1815a66b7f8c0e9d
[INFO] Status: Image is up to date for java:8
[INFO]  ---> d23bdf5b1b1b
[INFO] Step 2/5 : EXPOSE 8080
[INFO] 
[INFO]  ---> Using cache
[INFO]  ---> 75767466e0be
[INFO] Step 3/5 : ARG JAR_FILE
[INFO] 
[INFO]  ---> Using cache
[INFO]  ---> 2ecdd1234dc2
[INFO] Step 4/5 : ADD target/${JAR_FILE} /app.jar
[INFO] 
[INFO]  ---> 6169104a5073
[INFO] Step 5/5 : ENTRYPOINT ["java", "-jar","/app.jar"]
[INFO] 
[INFO]  ---> Running in 23596d4612b6
[INFO] Removing intermediate container 23596d4612b6
[INFO]  ---> 993715a0e72a
[INFO] Successfully built 993715a0e72a
[INFO] Successfully tagged com.naylor/dockerfile-maven-plugin:0.0.1-SNAPSHOT
[INFO] 
[INFO] Detected build of image with id 993715a0e72a
[INFO] Building jar: /Users/chenhd/code/DebrisApp_Springboot/debris-app/dockerfile-maven-plugin/target/dockerfile-maven-plugin-0.0.1-SNAPSHOT-docker-info.jar
[INFO] Successfully built com.naylor/dockerfile-maven-plugin:0.0.1-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  16.597 s
[INFO] Finished at: 2020-10-12T13:50:03+08:00
[INFO] ------------------------------------------------------------------------

重點分析一下此行後面的日誌:

--- dockerfile-maven-plugin:1.3.6:build (default) @ dockerfile-maven-plugin ---

Building Docker context /Users/chenhd/code/DebrisApp_Springboot/debris-app/dockerfile-maven-plugin

執行dockerfile-maven-plugin專案的docker上下文的構建

Image will be built as com.naylor/dockerfile-maven-plugin:0.0.1-SNAPSHOT

構建完成之後映象的名稱為:com.naylor/dockerfile-maven-plugin:0.0.1-SNAPSHOT

Step 1/5 : FROM java:8

dockerfile中一共定義了5步來執行構建,第一步是拉取java8的映象,如果本地沒有會從遠端倉庫中搜尋並下載下來

Successfully tagged com.naylor/dockerfile-maven-plugin:0.0.1-SNAPSHOT

成功打包了映象

引用

1:官網:https://github.com/spotify/dockerfile-maven

2:dockerfile參考:https://docs.docker.com/engine/reference/builder/

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