Given the way image building is likely to work, it is far simpler to
just use HHMM as a sequence number, instead of the XX in the current
implementation.
Also added ARCH to the filename.
The current name is in the format:
yymmdd_hhmm_release-flavor-arch
Added testing/.cache to .gitignore
Without the f it looks like it requires a number, not an arbitrary string. I would add it back to make this clear, that does not influence how we use that in our deployment. (It would be also nice to add value types to docstrings, but in this case, it would still be a bit confusing).