docker: add thin backend runtime image

[carl-codeorg]

First of a series of 4 PRs to introduce a working local k8s setup. Once the PRs are in we will be in a state where:

• The docker image hierarchy starts with a ruby-slim base image, and builds on top of that for more resource-intensive images that require apps build, curriculum content seeding, etc. This PR introduces the ruby-slim based image. It was developed in parallel with feat(docker): add cdo-base shared minimal runtime image #73851 and we will eventually update the k8s templates to use that as a base. For now, we can get the local k8s setup working on the image from this PR.
• The full local k8s setup can be run on a typical developer machine: Macbook Pro with 16GB of RAM, on Docker Desktop or Minikube.
• The default skaffold profile is backend-only. The first component to be moved into k8s will be ActiveJob workers, which don't require the frontend.

Other things this PR touches:

• Moves Bootsnap out of dev dependencies to take advantage of it in other envs.
• Adds some guards around Prosopite usage to account for cases where the gem isn't installed due to bundler config.
• Adds keepfiles as necessary to maintain the mimic feature; these might be removed in the future if we don't find it useful.

Links

• Jira:

Testing story

Deployment notes

Privacy and security

[nicklathe]

Seems kind of annoying to have to workaround prosopite here. Wondering if I look into a follow-up change that would only initialize prosopite in dev if a flag is set vs. always initing? Something like CDO.prosopite_enabled or something.

[carl-codeorg]

Yeah good point. It's possible we can remove this extra guard depending on how we ultimately decide to configure the RAILS_ENV and BUNDLE_WITHOUT for the k8s dev environment.

[nicklathe]

This LGTM, I like the new direction with defaulting to a base dev image.