Create a base image for helpers

[mook-as]

Is your feature request related to a problem? Please describe.
I'd like an image with helpers for run-time scripting; in this particular case, ruby + kubeclient gem. We'll probably need similar things as we move towards more native components as well.

Describe the solution you'd like
Some image (possibly created on OBS) that is basically the current stemcell.

Describe alternatives you've considered
More GitHub Actions + Dockerfile to generate docker images would work too. We'll need to have some place in the repo to put them though.

Additional context
This was spawned out of #1346 (comment).

[mook-as]

There's a https://hub.docker.com/r/splatform/base-ci but it is unclear where that comes from (there doesn't seem to be a repo for it).