Start freva service only AFTER docker is up

We've witnessed some hiccups with the systemd service of the freva sorl/vaul/db containers. Initially, I've told systemd that the freva service wants the docker service. Which is somewhat weaker than the requires flag. Meaning if docker fails the freva service will shutdown when the requires flag is enabled. The wants flag on the other hand would have no effect in this situation.

The original problem was that both the freva and the docker service were started at once and since the freva service was faster than docker it failed. Hence I've also told the service to attempt the start of the service after docker was started.

@k204229 it's just very few changes but I think it's a good idea to become familiar with the handling of the services ... that is systemd