Change entrypoint to work with read only mounts

[nickv2002]

The readme encourages use of mounting directories under /var/lib/dav/data but

1. It's bad practice to chown any volume/bind mounts to this directory because it might mess up permissions on the host machine
2. Trying to chown a readonly (ro) mount will fail and crash the container.

So I changed the chown commands to be more restrictive here to avoid those problems. Work fine for my own testing where I read only bind mount a directory under /var/lib/dav/data

This could possibly be version 2.5 but maybe worthwhile to combine with other changes. Thanks!