Docker: Support PGID/PUID Usage
In order to integrate better with watch folder permissions, please allow the setting of the Mayan EDMS processes' UID and GID via the convention of setting the environment variables "PGID" and "PUID".
Usage case: A Watch folder receives scanned documents from a multi-function printer via FTP and are subsequently owned by UserA with a UID of 1029. These documents may be sensitive and need to have permissions on the FTP server restricted from the UNIX group and 'other'. Mayan EDMS runs within its Docker container as a different UID. Mayan EDMS cannot read/remove files in the Watch folder because of differing UIDs. If the 'mayan' user's UID can be set, the container can run its processes and access the mounted Watch folder appropriately.