Image builder should use a version tag based on the source commit

[jlewi]

It would be nice to have a version tag with some semantic meaning for humans e.g. a tag based on a datestamp such as v20240129T075651. This way its easy for humans to tell if one version is newer than another.
Right now we use a version tag based on a timestamp of the build time. That's not great because its not stable.

We could however use the timestamp of the source commit because that's fixed, tied to the source commit and monotonically increasing.