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
Follow up to I6a564206dd81743deb1eb27eca7081bc333d7434 where the
"*-latest" Dockerfiles were introduced. msc, sgsn, sip, stp had
references to "nightly" instead of "latest" in their Dockerfiles.
Related: OS#3268
Change-Id: I1eab031038927043faf6ed8ed30bdf4d4d1624d5