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