Dockerfile: Update ordering of apt packages

Dockerfile

@@ -6,8 +6,16 @@ MAINTAINER "Ian Hinder <[email protected]>"
 ARG DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update -y && \
-    apt-get install -y python3 python3-pip python3-requests graphviz libgraphviz-dev curl && \
+    apt-get install -y \
+    python3 \
+    python3-pip \
+    python3-requests \
+    pkg-config \
+    graphviz \
+    libgraphviz-dev \
+    curl && \
     rm -rf /var/lib/apt/lists/*
+# Note: the ordering of these packages matters as libgraphviz-dev requires python3-pip
 
 COPY ./requirements.txt /app/requirements.txt