Enable user namespacing for static test containers

[aahlenst]

#1925 added static Docker containers that run SSH as root (and probably other things). We should configure user namespacing (and maybe static UID/GID) so that someone who manages to escape from the container does not become root on the host.

[sxa]

Potential good first issue for someone who knows their way around docker :-)
This relates to the docker files in DockerStatic ansible role