Checker: Support checker scripts as Docker containers

[F30]

In addition to #32 and #33 we might want to support launching Docker containers for checker scripts. This would make testing and deployment of checker dependencies easier.

One container per service, team and tick is probably too expensive, but one per service and team (and checker instance) could work. However, we would need some (enhanced or new protocol) to communicate with the containers.

[F30]

The containers should also be restricted to just allow network connections to teams, nowhere else.

[F30]

An alternative to an enhanced or new protocol would be running the containers permanently and exec()-ing into them for the actual checks.