[CI/CD] decouple regular commits from nightly releases

[JulienPeloton]

Rationale

The current CI pipeline has many disadvantages:

1. We have a single Docker image for everything
2. This image is huge due to scientific dependencies
3. integration tests take forever as we check everything at every commit

Proposal

The main problems come from raw2science which imports fink_broker.science, which in turns import fink_science modules, which in turns needs the whole Earth dependencies... It would be interesting to isolate this script, and have conditional imports depending on the commit strategy (regular vs nightly). In turns, this means we can define two images:

1. light image, with only core dependencies to test the skeleton
2. complete image for nightly releases