The actual code was included in the previous
97a50a23d2 by accident …
This commit only wraps the slightly more complex docker.yml condition
in GitHub's special ${{ … }} braces.
- run on every push to master
- build images from GitHub source instead of PyPI package
- build arm64 image (#5227)
- include more tags
- build date as 'YYYYMMDD'
- 'dev' for most recent build from master
- 'latest' for most recent release build