This allows easily changing to another osmocom repo base URL if the
default one is down.
Related: OS#4862
Change-Id: I8010b08f3dabacfb3c13a44eece6c7a490e0742e
Similar to what is already done with DISTRO, which points to given image
of ours based on name. This time we do the same with upstream images,
such as debian:stretch or centos:centos8.
This way, for instance calling docker_images_require
"osmo-bsc-latest-centos8" would try to build the
osmo-bsc-latest/Dockerfile file starting from a centos8 image.
Change-Id: I33cb21aa024396974559fd98f9f3c64e2c351eda
This is initialized to docker.io, keeping the default behaviour
if not specified. However, it allows us to specify a private
registry later on.
Related: OS#4839
Change-Id: I32d4ee6256033c809108c1b86cb6b6c58d880f49