Reverted the rootless container image path in `GITEA_APP_INI` from
`/etc/gitea/app.ini` to its default value of
`/var/lib/gitea/custom/conf/app.ini`. This allows container users to not have
to mount two separate volumes (one for the configuration data and one for the
configuration `.ini` file). A warning is issued for users with the legacy
configuration on how to update to the new path.
