docker/tests: Allow to run test sessions in a container and in parallel

These changes enable to run the docker tests in a container by using the docker bridge network gateway IP instead of the localhost domain name in the URLs that are requested during tests execution.

They also enable to execute multiple test sessions in parallel by patching the compose files at the beginning of each session to ensure service ports are bound to free ports on the docker host.

Depends on !298 (merged).

It also requires swh/infra/ci-cd/swh-jenkins-jobs!201 (merged) to be merged to update the Jenkins jobs.

I locally patched the swh-environment/docker-tests job configuration with the changes in it.