Java HelloWorld in Docker Hub w/ Multi-Arch Support

This is a test project to have Docker Hub use Automated builds to deploy both a amd64 and arm32v7 image of a minimal hello-world.

How it works: Docker Hub uses x86_64 machines for all build procedures, inhibiting native compilation of other architectures. By using QEMU in special Dockerfiles meant for other architecture builds and also using manifest-tools to stitch together all of the architecture builds under one release. We can have multiple architecture support just like the official Docker Hub Images.

##How to Replicate

1. Setup Docker Hub with automated builds on push of tags:

   ####Docker Hub Autobuild Settings

   Source Type	Source	Docker Tag	Dockerfile Location	Build Context	Autobuild
   Tag	/^[0-9.]+/	{sourceref}-amd64	amd64.dockerfile	/	True
   Tag	/^[0-9.]+/	{sourceref}-arm32v7	arm32v7.dockerfile	/	True

2. a pre_build hook will install qemu support docker run --rm --privileged multiarch/qemu-user-static:register --reset

3. a post_push hook will use manifest-tools to generate a collective manifest.

4. Push a release tag to the github repo (e.g. 1.1)

Resources / Credits:

• Heavy Lifting: ckula's docker-multi-arch-example for being the "base image" of knowledge for this topic.

• 2nd Heavy Lifter: guysoft's post-push hook file for automated releasing based on tags.

• manifest-tool

• Tibor Vass - Intro Guide to Dockerfile Best Practices

• Micheal Waltz - Cross Building and Running Multi-Arch Docker Images - alternative explanation to ckula's work.

#Caveats:

• Currently you cannot clone the repo on an ARM device and build with either dockerfile due to incompatible base images.
• AdoptOpenJDK images will not work with QEMU / building on amd64. But OpenJDK comes to the rescue with the support. Differences in their underlying images create this difference.