Move docker build to developer container

[krnowak]

Currently the built-in docker build command is deprecated in favor of the buildkit plugin. There's a PR (flatcar/scripts#2204) that adds the plugin to the generic image (or rather, to the docker sysext, which is a part of the generic image). Since the docker build is rather of a limited use without the build tools, it should rather be a part of the developer container.

The plan:

• Merge Add app-containers/docker-buildx scripts#2204, so generic image has a buildx plugin, no pesky deprecation messages are printed.
• Turn developer container into a sysext image.
• Start a deprecation period by announcing the move of the buildkit plugin to the developer sysext.
• Add a stub buildkit binary that prints a message about how to get a working docker build command or patch the docker binary to do it.

Notes:

Patching the docker binary might be preferrable despite having to maintain the patch, because if docker-sysext and developer container sysext both provide the buildkit plugin (a proper one and a stub one, respectively), then which one will end up on the merged filesystem depends on ordering of sysexts.