postgresql: Add PostgreSQL container instructions (Dockerfile and README)

postgresql: Add PostgreSQL container instructions (Dockerfile and README)

The $(echo /etc/postgresql*/postgresql.conf) crazyness in the
Dockerfile is because Docker chokes on:

  RUN echo host  all  all  0.0.0.0/0  trust >> /etc/postgresql*/pg_hba.conf

raising:

  /bin/sh: /etc/postgresql*/pg_hba.conf: No such file or directory

I'm not sure why it's not expanding the glob.  Perhaps Docker is
quoting redirection targets?  In any case, the $(echo ...) form works,
so that's what I'm going with for now.