Changes

debian-stretch-jenkins: move from osmo-ci.git
Put Dockerfile_osmocom_jenkins.amd64 ("osmocom:deb9_amd64") from 
osmo-ci.git here and give it a more descriptive name.
The image is used to run contrib/jenkins.sh scripts of most Osmocom 
projects. Move it to here, so we have all docker images in one place. In
the future, osmo-ci.git will use more images from docker-playground.git
in gerrit verification, such as debian-buster-erlang.
Related: OS#4345 Change-Id: I125ae8a6bcabbd1f485028c79b0abacda0622c3a

debian-buster-erlang: rename user to "build"
Match username of debian-stretch-jenkins, so we can use the same docker 
arguments for both images in gerrit-verifications.yml of osmo-ci.git.
Related: OS#4345 Change-Id: Ia3eaec6090c9652549b2850de74ee21730374bbd