**What this PR does / why we need it**:
Adds the ability to reference images by digest instead of tag. Useful
for images that are bundled in an OCI artifact, where the digest must be
used.

**Which issue(s) this PR fixes**:
Fixes #9440 

**Special notes for your reviewer**:

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [x] Documentation added
- [x] Tests updated
- [x] `CHANGELOG.md` updated
- [x] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

---------

Co-authored-by: Vladyslav Diachenko <82767850+vlad-diachenko@users.noreply.github.com>