Docker build and release as part of CI

[trevex]

The server and agent require a proper container.
The docker image should be built and pushed for each new tag on the master branch.

This can be more involved as the agent will be fairly interesting how to best interact next to wireguard in a container.
So we'll have to find out if wg and agent should be in the same container or if a sidecar solution via the optional agent daemonset
of #7 is more sensible.