CI - Only tag extra docker images on tag refs

[bfops]

Description of Changes

Previously, we would create an extra docker image for every git ref that was pushed. Now, we only do it for tags.

We had intended to remove the docker tags for git branches a long time ago, because it was cumbersome to map between "we're using the docker image of $BRANCH" to which exact commit was deployed. However, this merge-docker-images.sh script slipped through the cracks, so it was still creating docker tags corresponding to git branches.

API and ABI breaking changes

No code changes.

Expected complexity level and risk

1

Testing

• When I make the CI run on this branch, we successfully get all the sha docker tags (platform-specified, unified) but NOT a docker tag for the branch name
  • AMD64
  • ARM64 (this one merges the images)