ftl-build container improvements

[DL6ER]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide.
• I have checked that another pull request for this purpose does not exist.
• I have considered, and confirmed that this submission will be valuable to others.
• I accept that this submission may not be used, and the pull request closed at the will of the maintainer.
• I give this submission freely, and claim no ownership to its content.

How familiar are you with the codebase?:

10

---

1. Update ftl-build containers to debian:buster where applicable.
   Note: This sets the minimum supported kernel to Linux 3.2.0 (January 2012) for binaries built with these containers

   We do not update the containers:

   • armv4t: debian:buster dropped armv4t compatibility
   • armv6hf: There is a bug in debian:buster's cross-compilation libc missing fnctl64 for target armv6hf

2. We furthermore update x64_64-musl-container to latest alpine:3.12

3. Always clone and test-compile FTL's source from the protected branch special/CI_development. This ensures external contributions (and we ourselves) can see much more easily if changes/updates we do in the containers are good or not because the target architecture, FPU-requirements, minimum kernel versions, etc. are automatically checked during the container generation.

Points 1 and 2 make this PR superseding #14 which was not accepted before as there were more changes to be done to the building system than were not very obvious without a test compilation (which is included by default now due to no. 3 above).

[dschaper]

I'm sure we can get rid of GHR in the future as well, just upload using the CI's native methods. But that will be for another PR.