DEV: Use docker links to set password environment variable in the hub.

Also, yell at people if they use the default password.

examples/postgres/hub/Dockerfile

@@ -7,9 +7,6 @@ RUN apt-get install -y libpq-dev \
     && apt-get clean -y \
     && pip3 install psycopg2
 
-# Replace this with your own password.
-ENV JPY_PSQL_PASSWORD arglebargle
-
 RUN useradd -m -G shadow -p $(openssl passwd -1 rhea) rhea
 RUN chown rhea .