Docker pushes fail on tags

[Chris Markiewicz]

Example: https://app.circleci.com/pipelines/github/nipreps/smriprep/678/workflows/efef46c8-1cfc-4448-9afe-438e3cebc733/jobs/3269

#!/bin/bash -eo pipefail
docker tag nipreps/smriprep nipreps/smriprep:${CIRCLE_BRANCH#docker/}
docker push nipreps/smriprep:${CIRCLE_BRANCH#docker/}

Error parsing reference: "nipreps/smriprep:" is not a valid repository/tag: invalid reference format
Exited with code exit status 1
CircleCI received exit code 1

[Mathias Goncalves]

We should probably use the following heuristic:

1. check for CIRCLE_TAG
2. if none, check if CIRCLE_BRANCH has the docker/ prefix