Make the `tools/merge-docker-images.sh` script more generic

[bfops]

Description of Changes

This is a non-functional change to make the tools/merge-docker-images.sh script more generic/flexible.

This PR is a series of small (stacking) changes:

• add sanitize_docker_ref
• add IMAGE_NAME param
• make TAG and FULL_TAG into params
• tweak output and comments
• add set -u

API and ABI breaking changes

This does not change code.

Expected complexity level and risk

1

Testing

• [x] When I make the CI run on this branch, we successfully get all the docker tags (platform-specified, unified, and ref-tagged)
  • AMD64
  • ARM64 (this one merges the images)