Add graphviz to the Docker image

Dockerfile

@@ -5,7 +5,7 @@ ENV PYTHONUNBUFFERED=1
 WORKDIR /steering
 COPY requirements.txt .
 
-RUN apt-get update && apt-get install -y libmagic1 curl
+RUN apt-get update && apt-get install -y libmagic1 curl graphviz
 
 RUN pip install --upgrade pip
 RUN pip install --no-cache-dir -r requirements.txt