Is the ubuntu22-m2-cache docker image still used by anything?

### Description

`docker_image.yml` builds and pushes `apache/gluten:ubuntu22-m2-cache` (job `build-ubuntu22-m2-cache`), on every push to main that touches `dev/docker/ubuntu/*` and on the weekly Sunday cron. I cannot find anything that consumes the resulting image, so I would like to check whether it is still needed before someone spends more time maintaining it.

What I checked, at `cd1d1b993`:

- `grep -rn 'm2-cache\|m2cache'` over the whole repository returns three hits, all inside `docker_image.yml` itself: the job name, the `file:` path, and the `tags:` value.
- The `apache/gluten:*` images that workflows actually reference are `vcpkg-centos-7-gcc13`, `centos-8-jdk8`, `centos-9-jdk17`, `centos-8-jdk17`, `centos-9-jdk8`, `centos-9-jdk17-cuda13.1-cudf`, `vcpkg-centos-9`, `vcpkg-centos-8-gcc13`, `vcpkg-centos-7`, `vcpkg-almalinux-8-gcc13` and `centos-7-vcpkg-build-depends`. `ubuntu22-m2-cache` is not among them.
- No workflow uses it as a `container:`, and nothing in `.devcontainer/`, `dev/` or `docs/` mentions it.
- `git log -S 'ubuntu22-m2-cache' --all` returns exactly one commit, #11655, which introduced it. So it is not a case of a consumer having been added and later removed.

What each build costs: a Velox dependency setup (`builddeps-veloxbe.sh --run_setup_script=ON build_arrow`), then one full `dependency:go-offline` pass over the reactor per supported Spark version, then a push to Docker Hub.

So the question is whether it is consumed somewhere outside this repository, for example by a self-hosted runner setup, a fork, or as a developer convenience that is simply undocumented. If it is, a short note in `docs/developers/` would help, since nothing in the tree currently points at it. If it is not, the job looks like a candidate for removal.

Happy to move this to Discussions if that is the better venue.

For context on how this surfaced: #12903 fixes a typo in the same Dockerfile (`-Pspark-4` is not a profile id, so that stanza was silently resolving against the root pom defaults instead of Spark 4.0's), and #12902 edits the file as part of removing Spark 3.3 support. Neither depends on the answer here.

This description was written with the assistance of AI tooling (Claude claude-opus-5).

Start with docker_image.yml and the build-ubuntu22-m2-cache job; reproduce the repository-wide grep and git log -S checks described in the issue. Confirm with maintainers or known external users whether apache/gluten:ubuntu22-m2-cache is consumed outside this repository. Done means documenting a confirmed use in docs/developers/, or removing the unused job and image configuration.

Written by the indexing model from the issue text.