Allow setting PHP UID and GID via environment variables

Running this image and mounting /foo/bar/srv to /srv works fine, until I try to load a php website. I either have to set /foo/bar/srv to 777 or set /foo/bar/srv to UID 1000 manually, otherwise php files won't execute and won't be able to write files.

Having the ability to set the UID and GID to what I use for my Docker images currently would remove the manual steps to get PHP working (I spent tons of time trying to figure out why).

This is on a Synology NAS, tested with a test php file with phpinfo (for read/execute checks) and then with tt-rss, which failed because it required write perms (write checks).