latest docker tag is not a stable build

Ghost User
2018-12-14T16:43:03+01:00

### One-sentence description The `latest` docker tag should be a stable build. ### Why? The `latest` docker tag is the last pushed commit to any MR. It is better if `latest` is the last stable build (last git tag). ### Current implementation The `latest` docker tag is the last pushed commit. ### Suggested implementation Only push to `latest` if a git tag is build.