Add buildpack support

[philwebb]

Following on from #12545 we'd like to add buildpack support so that Docker images can be created easily from Maven/Gradle via a buildpack.

The following task need addressing:

• Port the pack CLI's build command to Java (Add cloud native buildpack module #19828)
• • Support lifecycle v6 (Update buildpack support for lifecycle v6 #19829)
• Create a new ImagePackager tools class (Create a new ImagePackager tools class  #19834)
• Add Maven plugin support (Add build-image support to the maven plugin #19830)
• Add Gradle plugin support ( Add buildImage support to the gradle plugin #19831)
• Create our own docker location for builder images and create CI for it (Use platform API builder image tags for default builder #20171)
• Add an integration test for the buildpack code that creates an image and tests it
• Zero the creation dates of the ephemeral builder (Update ephemeral builder metadata #20126)
• Use the spring boot version in EphemeralBuilder.updateMetadata(..) (Update ephemeral builder metadata #20126)
• Drop getDefaultDirsLayer and getStackLayer from EphemeralBuilder (Drop getDefaultDirsLayer and getStackLayer from EphemeralBuilder #19832)
• Test on a real Windows box
• Automatically setup $BP_JAVA_VERSION OpenJDK Buildpack env variable for JDK version (Configure OpenJDK buildpack to use the same Java version as used for compilation #20172)
• Update documentation (Document new Docker-related features #19868)
• Provide a publish option to push the image to a registry (including Apply multiple docker image tags when publishing images #20560)

[leobut]

I really love this feature and how it is coming along so far. Great work!
Just one question: Will it be possible to allow custom settings for the Docker image like for example additional ports or environment variables?

[snicoll]

@ControlTheBit that's really something to do in the builder. A buildpack it defines can contribute anything to the docker image. For instance the JMX builder can contribute the relevant JVM properties to JAVA_OPTS if JMX support is required. This is controller by a flag (BP_JMX) that you can set as an env variable in the Maven or Gradle plugin.

You can also come chat with the community on Gitter if you have more questions.

[philwebb]

I'm closing this one now since it's mostly complete. We'll deal with #20560 if we can.