NixOS: Run Docker containers as declarative systemd services (#55179)

* WIP: Run Docker containers as declarative systemd services

* PR feedback round 1

* docker-containers: add environment, ports, user, workdir options

* docker-containers: log-driver, string->str, line wrapping

* ExecStart instead of script wrapper, %n for container name

* PR feedback: better description and example formatting

* Fix docbook formatting (oops)

* Use a list of strings for ports, expand documentation

* docker-continers: add a simple nixos test

* waitUntilSucceeds to avoid potential weird async issues

* Don't enable docker daemon unless we actually need it

* PR feedback: leave ExecReload undefined