Switch from cross-compilation to QEMU-based emulation builds

[felddy]

🗣 Description

This change removes the tonistiigi/xx cross-compilation toolchain and instead relies on QEMU-based emulation for building multi-platform images. The Dockerfile is simplified by eliminating platform-specific commands (xx-apt) and dependencies.

Supported platforms are now limited to those that successfully build with the emulation approach. Platforms requiring additional changes can be added later via PRs.

Also:

• modifies how the --version flag determines the version of the container. Needed to support the 5.1.0+r1 tag.
• normalizes recommended :5 tag in sample docker-compose.yml and README.md.

💭 Motivation and context

pip was installing the wrong C extensions for some libraries. Specifically ephem.

• Resolves ephem package fails to install correctly for non-x86 architectures #395

🧪 Testing

On development box, production, and in CI.

Output from container built in GitHub Actions and pulled to an arm64 machine:

weewx@a3ca7e05579a:/home/weewx$ uname -a
Linux a3ca7e05579a 6.8.0-39-generic #39-Ubuntu SMP PREEMPT_DYNAMIC Sat Jul  6 02:50:39 UTC 2024 aarch64 GNU/Linux
weewx@a3ca7e05579a:/home/weewx$ find /opt/venv/lib/python3.13/site-packages/ephem/ -name _libastro*.so
/opt/venv/lib/python3.13/site-packages/ephem/_libastro.cpython-313-aarch64-linux-gnu.so

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• All new and existing tests pass.

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Add a tag or create a release.