Add environment variables to run gossa as unprivileged user

[jeslinmx]

Solves #31

This adds the following env variables:

• UID, GID: the uid and gid which gossa will run as
• HOST, PORT: sets the -h and -p flags (defaults to 0.0.0.0 and 8001)
• PREFIX: sets the --prefix flag (defaults to /)
• FOLLOW_SYMLINKS: sets the --symlinks flag (defaults to false)
• SKIP_HIDDEN_FILES: sets the -k flag (defaults to true)
• DATADIR: the directory within the container which will be shared by gossa

[pldubouilh]

thanks a lot for this - yep it all makes sense - I'll have a look tomorrow :)