Testing: Run rucio/rucio integ tests for PRs in rucio/containers

[rdimaio]

for PRs in rucio/containers, we could pull rucio/rucio:master and run the integ tests with the changes in the PR, to catch possible errors in advance

[rdimaio]

This syntax can be used https://stackoverflow.com/a/63822945 to conditionally do something depending on the push path in the PR. This way, we could make the GH workflow only build the containers that have been changed in the PR, and pull the remaining containers from the latest release on Docker Hub (similarly to how we do integ tests in rucio/rucio)