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