Update build workflows to use image caching, multi-arch, auto-tag

[spwoodcock]

Fixes #837 Fixes #786

Tidy up:

• Move scripts dir and (remote) josm dir to contrib.

Update to workflows:

• Build docs from development, instead of main.
• Add caching to builds using github registry caching & buildx for builds.
• Make images multi-arch by default (run on AMD + ARM (Apple M chips).
• Auto-tag images using semver:
  • Images built during push will simply be tagged after the branch.
  • Images build from a tag or release will be tagged accordingly.
• .env set by all vars and secrets in environment (non-selective).

Progress towards #920

Note (important)

• As part of this PR I added image caching for images during the pytest run.
• This actually decreases performance slightly over directly pulling images every time (30s to 1m extra run time).
• However, it is a worth tradeoff. Many container registries impose rate limiting (docker.io), so this will help to prevent tests failing due to rate limiting (from many consecutive pytest runs) if we have a lot of PRs being working on.