Fix docker image not building

Also adds a line in the readme to use the mistral client inside the
mistral container.

Change-Id: Ie09b06f682679b9a90d18b8796c47b69e91d14cf
Closes-Bug: 1782531

tools/docker/DOCKER_README.rst

@@ -63,6 +63,14 @@ for example::
                -f tools/docker/docker-compose/mistral-single-node.yaml \
                -p mistral up -d --build
 
+Running the Mistral client from the Docker Compose container
+------------------------------------------------------------
+
+To run the mistral client against the server in the container using the client
+present in the container::
+
+  docker run -it mistral_mistral mistral workflow-list
+
 Configuring Mistral
 -------------------
 

tools/docker/Dockerfile

@@ -8,6 +8,7 @@ LABEL name="Mistral" \
 RUN apt-get -qq update && \
     apt-get install -y  \
       libffi-dev \
+      libpq-dev \
       libssl-dev \
       libxml2-dev \
       libxslt1-dev \
@@ -22,7 +23,8 @@ RUN apt-get -qq update && \
       libuv1 \
       libuv1-dev && \
       curl -f -o /tmp/get-pip.py https://bootstrap.pypa.io/3.2/get-pip.py && \
-      python /tmp/get-pip.py && rm /tmp/get-pip.py
+      python /tmp/get-pip.py && rm /tmp/get-pip.py && \
+      pip install --upgrade pip
 
 RUN pip install pymysql psycopg2 py_mini_racer