Multiple build system improvements

[wohali]

Overview

Docker builds were failing locally for a couple of reasons:

• Cuda Ubuntu Arm64 and PPC64LE Image builds failing on VM with Buildkit Only moby/buildkit#1943 was causing buildx-based cross-arch builds to fail.
  • To fix, this PR replaces multiarch/qemu-user-static with tonistiigi/binfmt which is directly supported by a core Docker maintainer. It uninstalls all qemu emulators, then replaces them with Docker-supported qemu emulators.
• Keyservers were timing out on builds
  • Added hkps://hkps.pool.sks-keyservers.net to the list of keyservers in all relevant Dockerfiles
  • Install gosu and tini directly from Debian now that it is officially packaged
    • This has the bonus side effect of making the Dockerfile easier to read and faster to build!
    • To get this working for 2.3.1 I had to bump that image to buster.

These additional cleanups were performed:

• Removes junkfile md from root directory
• Removes the obsolete 1.7.2* and 3.0.1 definitions. They will live on in git history for anyone who wants them.
• Removes support for building ppc64le by default. This will be re-added once proper HW is re-added to the CI infra.

Testing recommendations

./build.sh buildx 3.1.1

(This will succeed but fail at the push part unless you have credentials to push to the apache Docker Hub org.)

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;