Fix extracting git tag in Pipeline and push v0.20.0 to Docker Hub again

[ccremer]

The latest 0.20.0 release job failed to publish to Docker: https://github.com/oetiker/znapzend/runs/527798365

This PR should fix this for the future, examples here:

$ GITHUB_REF=refs/tags/0.3.1-rc1asdfasdf
$ echo ${GITHUB_REF#refs/heads/} # this one doesn't work, that's why the Docker build failed in the job
refs/tags/0.3.1-rc1asdfasdf
$ echo ${GITHUB_REF##*/}
0.3.1-rc1asdfasdf

After that, either re-tag v0.20.0 to this merge-commit, or bump to v0.20.1, or just cheat a bit locally with

docker pull oetiker/znapzend:master
docker tag oetiker/znapzend:master oetiker/znapzend:v0.20.0
docker push oetiker/znapzend:v0.20.0

since the master tag got built and pushed successfully and it's currently the latest merge commit in master branch.

Whatever you prefer ;)

[oetiker]

thanks ... done