Add Environment Variable for EXTRA_ARGS

[driverpt]

e.g.: `-e POSTGRES_EXTRA_ARGS="-c 'max_prepared_transactions=100' -c 'max_connections=100'"

[yosifkit]

This is already available via command (as now mentioned in the docs with docker-library/docs#1095) so an environment variable is not necessary.

$ docker run -d --name some-postgres postgres:10.1 -c 'max_prepared_transactions=100' -c 'max_connections=100'
08f5b3264de1d4124440b4fbc17eb300caa434caf2f33cbbb44ba20a88adbce9
$ docker run -it --rm --link some-postgres postgres psql -h some-postgres -U postgres
psql (10.1)
Type "help" for help.

postgres=# show max_prepared_transactions;
 max_prepared_transactions 
---------------------------
 100
(1 row)

postgres=# show max_connections;
 max_connections 
-----------------
 100
(1 row)

postgres=#