Pass environment variables to docker container

This change allows arbitrary environment variables to be passed to the
underlying docker container. Useful when using postgres docker
containers which allow for additional configuration on top of the
official postgres image.

See https://github.com/mcolyer/docker-postgres-wale for an example.

README.md

@@ -24,7 +24,7 @@ dokku plugin:install https://github.com/dokku/dokku-postgres.git postgres
 ```
 postgres:clone <name> <new-name>  Create container <new-name> then copy data from <name> into <new-name>
 postgres:connect <name>           Connect via psql to a postgres service
-postgres:create <name>            Create a postgres service
+postgres:create <name> <env=val>  Create a postgres service with environment variables
 postgres:destroy <name>           Delete the service and stop its container if there are no links left
 postgres:export <name>            Export a dump of the postgres service database
 postgres:expose <name> [port]     Expose a postgres service on custom port if provided (random port otherwise)