This removes the upstream GitHub actions since we will be building our
own.

This updates the `Dockerfile.template` to build Ruby with `jemalloc` on
both Debian and Alpine. It also keeps `jemalloc-dev` installed so native
gems can compile against Ruby’s jemalloc headers after the build.

This adds a GitHub action to update `versions.json` will the latest Ruby
versions and regenerate the Dockerfiles using the `Dockerfile.template`.

It will run on a daily schedule and can be trigged manually from the
actions tab.

A separate action will be added later to build and push the images on a
less frequent schedule.

This introduces a GitHub action that utilizes the auto-generated
Dockerfiles to build, test, and push ruby-jemalloc Docker images to GHCR
for our supported Ruby versions and Linux distributions.

The action is scheduled to run weekly on Monday mornings, following the
job that regenerates the Dockerfiles to account for new Ruby updates.

Note: GHCR was recommended by DevOps over DockerHub to reduce the number
of container registries we use. It also provides good support for
private images being accessed locally as well as in other GitHub Action
pipelines.

We do not want a failure in one version build to cancel all of the other
in progress builds, especially given how long each takes.

The builds are core-dumping with ubuntu-24.04 (20250126.1.0)

After considering the security and operational complexities with using
GHCR we have decided to push to Docker Hub instead. The images will be
pushed to `ltvco/ruby-jemalloc` and be public. We will phase out the old
`ltvco/ruby`.

The previous approach of using the private `aws-secrets-loader` does not
work as this repo is a public fork. This instead auths using GitHub
Actions secrets.

This applies the results of running `./versions.sh` followed by
`./apply-templates.sh` to regenerate the Dockerfiles based on the
updated `Dockerfile.template` which now includes `jemalloc`.

This is effectively what the GitHub action will run on a regular basis.

This adds trixie and alpine3.22 to `build-test-push-images.yml` to align
with the non-EOL variants supported in the default `ruby` image.

This reverts several commits related to changing the registry from GHCR
(how this was initially designed) to Dockerhub, as we had some issues
at the time related to enabling public GitHub packages.

We are now focused on transitioning off of Dockerhub, so we are
returning to the original strategy.

Reverts:
- 6f2559a
- 5cca364

- Sets `contents: write` for the job that rebuilds and pushes updated
  Dockerfiles
- Sets `contents: read` for the job that defines the matrix and kicks
  off the individual image build/push jobs

This job was failing after restructuring the repo to use `jemalloc`
as the default branch. The workflow no longer hardcodes a branch name
and instead uses `github.ref_name`, which resolves to the correct branch
(including the default branch for scheduled runs).

The push command uses `HEAD:<branch>` because Actions checks out a
detached HEAD. This ensures the current commit is pushed to the remote
branch even without a matching local branch.

- Enables auto-reviews
- Removes placeholder custom instructions
- Removes default base branch config which does not apply
- Disables third-party tools that do not apply