This commit updates the directory permissions to be more compatible when
running the image without root f.e. on OpenShift or when specifying it
when running with `docker run --user www-data:root ...`.
It adds detection logic to the entrypoint script as sudo is not always
allowed.

This change in directory permissions was also proposed by the official
documentation, see https://github.com/nextcloud/documentation/commit/22e2530.

The `chown` before the volume definition is needed to prepare the volume
as it inherits the permissions.

refs https://github.com/nextcloud/docker/issues/107