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
Create centos8-build similar to debian-stretch-build, as preparation for
ttcn-3 tests on centos 8.
Related: OS#4564
Change-Id: Ide1e88c92b7ac301f0ea8ca110c1b2eb43f9d5ad