One of the things that you might want to do if you are using Docker and Java is to build the image from a Dockerfile in your build tool, such as Maven or Gradle. In this post, I am going to show you how to do it in both cases. I am going to assume that you have the de-facto project layout, having the Dockerfile file at the root of the project.

Maven

There are several Maven plugins that can be used for building a Docker image in Maven, but one of the most used is the fabric8-maven-plugin.

To start, you need to register and configure the plugin in pom.xml:

<build>
  <plugins>
    <plugin>
      <groupId>io.fabric8</groupId>
      <artifactId>docker-maven-plugin</artifactId>
      <version>0.16.7</version>
      <configuration>
        <images>
          <image>
            <name>arquillian/age-checker:${project.version}</name>
            <build>
              <dockerFileDir>${project.basedir}</dockerFileDir>
            </build>
          </image>
        </images>
      </configuration>
    </plugin>
  </plugins>
</build>

In the configuration section, you set the image name and the directory where the Dockerfile is located.

Any additional files located in the dockerFileDir directory will also be added to the build context. Since Dockerfile is the root of the project, the target directory is added too. The problem arises because this plugin uses the target and Docker to generate the build, and if you try to build it, you'll get next exception: tar file cannot include itself. To avoid this problem, you need to create a .maven-dockerignore file specifying which directory must be ignored at the same level as the Dockerfile:

target/docker/

And that's all! After that, you can do:

mvn package docker:build

Notice that this plugin honors Docker environment variables such as DOCKER_HOST, DOCKER_CERT_PATH, etc., so if your environment is correctly configured, then you don't need to do anything else.

Gradle

There are several Gradle plugins that can be used for building a Docker image in Gradle, but one of the most used is gradle-docker-plugin.

To start you need to register and configure the plugin in build.gradle:

buildscript {
    repositories {
        jcenter()
    }

    dependencies {
        classpath 'com.bmuschko:gradle-docker-plugin:3.0.3'
    }
}

apply plugin: 'com.bmuschko.docker-remote-api'
import com.bmuschko.gradle.docker.tasks.image.DockerBuildImage

docker {
    if (System.env.containsKey('DOCKER_HOST') && System.env.containsKey('DOCKER_CERT_PATH')) {
        url = System.env.DOCKER_HOST.replace("tcp", "https")
        certPath = new File(System.env.DOCKER_CERT_PATH)
    }
}

task buildImage(type: DockerBuildImage) {
    dependsOn assemble
    inputDir = project.rootDir
    tag = "arquillian/game-service:${project.version}"
}

In the case of Gradle, you need to configure Docker host properties since the plugin does not honor Docker environment variables. You need to configure them in docker {} block.

Finally, you create a task of type DockerBuildImage in which you set the Dockerfile root directory using inputDir attribute and image name using tag attribute.

Conclusion

In this post, you've seen different ways of doing the same in two different build tools, which is building a Docker image from a Dockerfile. Notice that these plugins also allow you to define the Dockerfile content as a configuration field, so you are not creating a Dockerfile file, but specifying its content inside the build tool. You can read more about this feature here in the case of Maven plugins and here in the case of Gradle.