ci: fix missing tag on containers on git tag pushes

ci: fix missing tag on containers on git tag pushes

Non semver tags like v26 were not included in metadata output, so image
pushes only produced sha- tags. Add tag reference metadata so tag pushes
publish a matching container tag.

  ##[warning]v26 is not a valid semver. More info: https://semver.org/
  ##[group]Docker tags
  ghcr.io/openwrt/buildbot/buildmaster-v3.11.8:sha-f2c82f2

Fixes: 93918cc2e225 ("ci: add support for testing PR containers")
Signed-off-by: Petr Štetiar <ynezz@true.cz>