Install libpq-dev so psycopg2 can build correctly

docker/Dockerfile

@@ -5,6 +5,8 @@ ENV PYTHONUNBUFFERED=1
 ENV CRYPTOGRAPHY_DONT_BUILD_RUST=1
 WORKDIR /opt/healthchecks
 
+RUN apt update && apt install -y libpq-dev build-essential && rm -rf /var/apt/cache
+
 COPY requirements.txt /tmp
 RUN \
     pip install --no-cache-dir -r /tmp/requirements.txt && \