Update the CC7 Developer image to an Alma9 based developer image

[d-ylee]

Impact of the new feature
We currently still have the CC7 docker image (gitlab-registry.cern.ch/cmsdocks/dmwm:wmcorepy3_tests) that can be used by developers to run unittests, as described here: https://cms-wmcore.docs.cern.ch/development/setup-wmcore-unittest/#setting-up-wmcore-unit-tests. The CC7 includes the WMCore code, packages required for testing, and MariaDB and CouchDB. However, our current WMAgent deployment method is using Alma9-based images and have MariaDB and CouchDB in separate containers. Our current Jenkins testing pipelines also uses Alma9-based images. This new feature would replace the CC7 image with an Alma9-based setup similar to how we deploy WMAgent.

Is your feature request related to a problem? Please describe.
This feature would allow developers to have an environment set up more closely resembling our deployment methods.

Describe the solution you'd like
Create a developer-use image for running unit tests locally, which would more closely resemble our Alma9 deployments and testing.

Describe alternatives you've considered
Have developers continue using the CC7 image.

Additional context
A part of meta issue #11978

[amaltaro]

Thanks Dennis! I added this ticket as a subtask in the meta #12261

[d-ylee]

I have pushed changes to a new repo under my account here: https://github.com/d-ylee/wmcore-devcontainer

With this setup, I can run unit tests that:

• Connect to Integration Rucio
• Connect to a local MariaDB, CouchDB

[d-ylee]

@amaltaro I have updated the repository with PLATFORM as an env variable and updated MariaDB to 10.6.19.