Buildx: Build Cache and Platforms

[cimnine]

Related Issues: #468, #306

TODO

• investigate error: failed to solve: rpc error: code = Unknown desc = docker exporter does not currently support exporting manifest lists
• docker-compose up on a Raspberry PI (or other ARM computer)

New Behavior

We were using Docker Buildx in our scripts since a while.
We used it via docker build by exporting the respective ENV var, DOCKER_BUILDKIT=1.

This change switches to using docker buildx commands explicitly.
This allows us to use more advanced features:

• Exporting and Importing the build cache
• Cross-compiling for other platforms, e.g. linux/arm64
• Pushing straight after building (not used in GH Actions)

Contrast to Current Behavior

Currently only the most basic features of Buildx can be used, because Docker does not expose the advanced features through the docker build command.

Discussion: Benefits and Drawbacks

My main hope is that this change speeds up builds. Because the build-cache can be exported, GH Action should be able to restore the build-cache the next time it attempts a build. This should even be true for branch builds, i.e. builds for PRs.

Changes to the Wiki

I don't think that changes are required.

Proposed Release Note Entry

## ARM64 Builds #474

We now build and ship an ARM64 build of the image. This was made possible by switching to the Buildx build system.

Double Check

• I have read the comments and followed the PR template.
• I have explained my PR according to the information in the comments.
• My PR targets the develop branch.

[cimnine]

I will re-open this later