Add in declarations for username and password variables (docker/docker)

The script was failing to create the user properly when I was trying to mount the data directory using the volume parameter; my logs were showing this:

    WARNING: enabling "trust" authentication for local connections
    You can change this by editing pg_hba.conf or using the option -A, or
    --auth-local and --auth-host, the next time you run initdb.

    PostgreSQL stand-alone backend 9.3.5
    2014-07-27 03:28:20 UTC ERROR:  syntax error at or near "WITH SUPERUSER" at character 14
    2014-07-27 03:28:20 UTC STATEMENT:  CREATE USER  WITH SUPERUSER PASSWORD '';

I just added the variables to the start.sh script, I'm not sure if the values were supposed to be coming in externally somehow or what.