[Chapel Merge] Unpin Github CI Dockerfile dep versions, build periodically

Branch: refs/heads/main
Revision: 1da64b74cee8d1db2e315f3e949924417ac02a0b
Author: arifthpe
Link: Unpin Github CI Dockerfile dep versions, build periodically by arifthpe · Pull Request #28807 · chapel-lang/chapel · GitHub
Log Message:
Unpin Github CI Dockerfile dep versions, build periodically (#28807)

In the Dockerfile used to build the container our Github Actions
workflows run in, use the :latest (LTS) version of the ubuntu base
image and the default version of chpl dependencies from its repos.

Also includes:

Similarly to Unpin Dockerfile base image and package versions by riftEmber · Pull Request #26713 · chapel-lang/chapel · GitHub, this
makes it so we don't have to worry about manually updating this image
(like in Update CI image to a newer OS/package depedencies by jabraham17 · Pull Request #27463 · chapel-lang/chapel · GitHub). This should
pretty much always give us a "reasonable" version of the OS and our
dependencies, and I think it's unlikely that latest LTS Ubuntu will
outrun what we support -- if it did, we'd have to update to accommodate
it anyways.

Has the nice side effect of reducing the image size from 549MB to 491MB
(in my local build).

[reviewed by @jabraham17, thanks!]

Compare: Comparing 10ff2aeaaf52a683effd3b7144b805900e97c4e8...393bafa38e6298f88c8e3a8a8370f97500caa708 · chapel-lang/chapel · GitHub

Diff:
M .github/workflows/build-CI-container.yml
M util/packaging/docker/github-ci/Dockerfile
https://github.com/chapel-lang/chapel/pull/28807.diff