The docker container on PR action is broken in case the PR is not opened
within the Gluon project.

Remove the build on PR so the pipeline does not fail for third party
PRs.

Signed-off-by: David Bauer <mail@david-bauer.net>

This adds a workflow to push the build-container described in Gluons
Dockerfile to the freifunk-gluon organizations container registry.

This will create a image

 - For every push to master
 - For every tagged version
 - For every PR against master

Signed-off-by: David Bauer <mail@david-bauer.net>