Update the "Build docker images" GitHub Actions workflow to use
`docker/metadata-action` to generate docker image tags, instead of a
custom shell script.
Signed-off-by: Henry <97804910+henryclw@users.noreply.github.com>
PyNaCl's recent 1.5.0 release on PyPi includes arm64 wheels, which means our
arm64 docker images now build in a sensible amount of time, so we can skip the
amd64-only build.
