Install the postgres dependencies

Dockerfile

@@ -1,12 +1,12 @@
 FROM python:2-alpine
 
-RUN apk add --no-cache --virtual .nacl_deps build-base libffi-dev zlib-dev openssl-dev libjpeg-turbo-dev linux-headers
+RUN apk add --no-cache --virtual .nacl_deps build-base libffi-dev zlib-dev openssl-dev libjpeg-turbo-dev linux-headers postgresql-dev
 
 COPY . /synapse
 
 # A wheel cache may be provided in ./cache for faster build
 RUN cd /synapse \
- && pip install --upgrade pip setuptools \
+ && pip install --upgrade pip setuptools psycopg2 \
  && mkdir -p /synapse/cache \
  && pip install -f /synapse/cache --upgrade --process-dependency-links . \
  && mv /synapse/contrib/docker/* / \