Spin up DAViCal docker container for testinga

[tobixen]

Should follow the same framework as the other docker containers, including being spun up and tested by the github CI

[tobixen]

By average I spent around 6 hours fighting for each of the docker containers I've been spinning up so far. I think the DAViCal is probably not the most difficult one (although it does need a database backend), but I still expect to spend 4 hours fighting with this one.