Adjust the Dockerfiles, so 'docker_images_require osmo-mgw-master-centos8'
etc. result in a centos8 based image. centos8-build already configures the
Osmocom nightly repository, so we only need to configure it for
debian-stretch.
Related: OS#4564
Change-Id: I6579748056ce0505304378b342a698b98c77fd18
This way we can run the container with osmo-remsim-bankd + ifd-vpcd,
i.e. with a simulated smart card reader behind the bank.
Change-Id: I463eca0f66197b409e00028e75e9501de620cc8b