ref #1: Move package retrieval before source copy so that it can be cached

Dockerfile

@@ -17,10 +17,10 @@ RUN useradd -m $USERNAME && \
         usermod  --uid $uid $USERNAME && \
         groupmod --gid $gid $USERNAME
 
-WORKDIR /home/$USERNAME
-COPY src /home/$USERNAME/src
 RUN apt-get update && apt-get install -y \
       python-catkin-tools bash
+WORKDIR /home/$USERNAME
+COPY src /home/$USERNAME/src
 RUN rosdep install --from-paths . -r -y
 RUN catkin config \
       --extend /opt/ros/melodic && \