[Docker] Use `SELECT 1` to wait for database

This removes the need to have host and port as environment variables (we might get it in a YAML file from a K8s ConfigMap, or some such). Also, it ensures that the system answering is a database rather than an IoT Waffeleisen with a bad taste in port selection.

Dockerfile

@@ -31,6 +31,7 @@ RUN apt-get -y update && \
 	libpq-dev \
 	libssl-dev \
 	netcat-openbsd \
+	postgresql-client \
 	yarnpkg && \
     eatmydata pip install uwsgi django-compressor