[Feature] Add support for env PUID and PGID

[isitLoVe]

Please add support to run the container as the user/group specified in environment variables (PUID and PGID).

Using a QNAP with container station it is not possible to specify the -u parameter, so the container runs as admin. All downloaded files have the admin permission and a normal user cannot move them.

Here is a similar container supporting the two environment variables: https://github.com/gangefors/docker-airdcpp-webclient/