Separate WMAgent docker files between base vs application image

[amaltaro]

Impact of the new feature
WMAgent containerization

Is your feature request related to a problem? Please describe.
This issue is a result of observations made when completing this GH issue:
#8797

where a separation of the docker images will bring us the following benefits:

• OS libraries will be installed only in the base image
• making WMAgent images more reproducible and avoiding underlying libraries updates between patch releases
• making the docker build process faster by installing only WMAgent application related packages in the final image

Describe the solution you'd like
Pull request has already been created :-)

But the expected solution is to separate the wmagent Dockerfile into two images:

1. wmagent-base: installing the OS-based dependencies for WMAgent
2. wmagent: running the actual wmagent deployment, including all the needed scripts.

Describe alternatives you've considered
Eventually we might consider breaking this further into smaller images.

Additional context
Belongs to the following meta-issue: #11314