bug(ci): fix docker release tag

[fynnfluegge]

In the current docker build and publish workflow the release tag from $GITHUB_REF_NAME is not correct:

 - name: Extract release tag
   id: get_tag
   run: echo "RELEASE_TAG=$GITHUB_REF_NAME" >> $GITHUB_ENV

The result is that the RELEASE_TAG has the value main as shown in the logs here:

The reason might be that the Docker Build and Push pipeline is triggered by Release Build and not by the tag creation while Release Build itself is triggered by a tag creation.

How to test

It is hard to test without pushing a tag. A scenario would be a dedicated test repository with a similar minimal pipeline setup and test how to retrieve the release tag.