Multiplatform build

[niccokunzmann]

This makes a start at fixing #216.

Changes:

• Add buildx option to Makefile
• Add buildx install to github action
• Add PLATFORMS variable to github actions
• Document building with make in README

I will check the platforms now...

• linux/arm64

• linux/amd64

  134.1 Processing triggers for libc-bin (2.31-13+deb11u11) ...
  134.3 Segmentation fault (core dumped)
  134.5 Segmentation fault (core dumped)
  134.5 dpkg: error processing package libc-bin (--configure):
  134.5  installed libc-bin package post-installation script subprocess returned error exit status 139
  

  libc-bin segfaults ( script subprocess returned error exit status 139 ) ( qemu: uncaught target signal 11 (Segmentation fault) - core dumped )  docker/buildx#314

• linux/arm/v8

• linux/arm/v7

• linux/arm/v6

[niccokunzmann]

The alpine build works.

[ImreSamu]

Thank you for the PR.

When I tested the QEMU solution some time ago, I ran into a few emulation issues.
Because of that, I am much more cautious with this technique.
However, I might need to re-evaluate some things since then.
#393 (comment)

[niccokunzmann]

@ImreSamu Thanks for the link, I had a look at your PR. You seem to be well versed in what to change in the Docker files. I do not know that so much...

Since #216 mentions this: #216 (comment)
It seems builds are already been done. I wonder why alpine builds work but buster builds do not under arm64 for this configuration I have here... Official alpine images would be a start or adding a link to the other Repository mentioned here in the README. I can check more about why the buster build fails...

[niccokunzmann]

This now only adds the buildx possibility to build the image for another platform. The CI is not affected. I will create a new PR for the CI.

Reason: #216 (comment)