Prefer compose config over building containers
There doesn't seem to be a need to prefer building custom containers, just to add a config file, a need that is easily solved with compose configs. Configs are still managed through git, just no new container needs to be built, slightly simplifying things.
While here, fix a cron-race-condition with regards the app container.
This was on purpose due https://github.com/nextcloud/docker/pull/538#issuecomment-439074124
@J0WI fair nuff; but it's 5 years later ... having bad practices in examples cause of windows is a poor excuse :D A note should be enough maybe?
Readme needs to be updated as well
superseded by https://github.com/nextcloud/docker/pull/2405