Expand the repro documentation with missing bits:
- The Dockerfile needs to be regenerated with the correct group for title annotation to ensure reproducibility.
- The CI_COMMIT_SHA of the original pipeline needs to be honored in the Dockerfile.
Also, set the timezome to UTC in Makefile and scripts to ensure consistency in the generated dates / timestamps (e.g. ARCHIVE_SNAPSHOT / SOURCE_DATE_EPOCH), regardless of the timezone of the environment. Otherwise, someone rebuilding the image locally can unexpectedly end up with a different value for those if the system uses a different timezome.
There's actually a dedicated flag to ignore image name annotations specifically so we don't have to ignore the other attributes with the `--semantic` flag.
