ci: create initial gitlab-ci jobs #1

This PR adds two simple jobs for building libpEpAdapterfor Debian 10 and CentOS 8.

They will pull down docker images containing the dependencies needed for
libpEpAdapter, then proceed with the build. Upon successful compilation,
an image containing the ouput (and dependencies) will be pushed to a
docker registry on the CI infrastructure.

Note about the docker images in use:
Currently they pull the latest docker images containing pEpEngine,
Sequoia, and other dependencies. We want to specify the exact required
versions in future updates to the jobs.

You can see it building successfully here on the Gitlab:

https://pep-security.lu/gitlab/dvn/libpEpAdapter/-/pipelines/583

For the CI to run we will need to keep a mirror of this repo on our Gitlab server.

I've set one up, and it's located here: https://pep-security.lu/gitlab/cid/mirrors/libpepadapter

Issues, Merge Requests, Forking, etc. have all been disabled.

I haven't finished setting up the automatic mirroring, but that will be done shortly.

So when new commits come in here, they will appear over there too, and then we can watch the CI pipelines run: https://pep-security.lu/gitlab/cid/mirrors/libpepadapter/-/pipelines