make owncloud cli occ usable via docker exec

[martingabelmann]

as mentioned in #67 . it should be documented at the hub page? example usage: docker exec oc_container occ app:check calendar

[lazyfrosch]

Yes, we should add a section to docs, when the addition gets accepted

[tianon]

Sorry for the delay! I think we'd have better parity with the documentation for this tool (https://doc.owncloud.org/server/9.0/admin_manual/configuration_server/occ_command.html) if we consider this a strictly documentation issue instead.

For example, the following have nice symmetry IMO:

$ sudo -u www-data php occ status # example from the docs
$ docker exec -u www-data my-owncloud php occ status

[martingabelmann]

I agree. Minimal complexity but the same result (since the docs should contain something about this anyway).