docker: Install libpq-dev to fix psycopg2-binary build

docker: Install libpq-dev to fix psycopg2-binary build

psycopg2-binary fails if pg_config isn't installed, which is provided by
libpq-dev.

This seems strange to me since psycopg2-binary suggests that
you use psycopg2-binary instead (of itself) if you don't want to build
psycopg2 so you wouldn't need pg_config, which is very confusing.

It's possible that psycopg2-binary only needs to compile itself on
non-x86 platforms, since I hit this on ppc64le.

Anyway, it works when this is added.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>