CI with Docker

Hey All,

I just created an MR for the point of having a more robust and stable CI using an image that is not based on ATLAS analysis images. The technical details are all contained within the CI file, of course, but to give a bit of summary.

The image for FASER DAQ is created here - https://gitlab.cern.ch/faser/docker. This repo is set up to also build offline images when we/they (@dcasper and @keli) get to that point.

The DAQ image depends on the daqling image built here - https://gitlab.cern.ch/ep-dt-di/daq/daqling/blob/develop/etc/Dockerfile - but pulls the master/latest version of this image. This means it may be a bit unstable. @engamber and I spoke about this and there are thoughts to create tags of this. But for the moment, this should be fine. It should be noted that this daqling image is a bit misleading if we call it that. It is actually just the dependencies of daqling and we still need to rebuild the entirety of that repo in our CI job.

Currently this CI pulls the master version of the faser-offline image, but it can easily be configured to use tags. And if you create a tag of the faser-docker image then it will automatically push such an image.

@aagaard , when reviewing/approving this eventually, we will need to add credentials to the CI/CD Variables so that the automatic DOxygen builds can push to EOS.

Cheers, Sam