ci : create git tags for released docker images (#16008)

* ci : create git tags for released docker images

When releasing a docker image for build number X, we should also create
the corresponding git tag. This allows users to easily checkout the
corresponding source tree for given docker image.

* Update .github/workflows/docker.yml

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

* Update .github/workflows/docker.yml

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

* Apply suggestion from @CISC

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

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Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>