[CI] Ok, this should be the CI we want.

Basically :
- When you push a Dockerfile, all the pipeline will be triggered, except the push-prod stage.
- When you push a docker-compose, only the security checks will be done, based on the latest built image. It is in that case useless to rebuild the image!
- When you push a clair-whitelist, only the Clair stage will be done. Same reason as above.
- When you push something else (e.g. a config file), you may trigger any step you want by hand, for flexibility. No more dumb "Trigger CI" commits!
- When you push something else (not in a pica-* folder), nothing happens and you cannot trigger anything. This is expected.