add empty linen to Dockerfile

Author: Nils-ChristianIseke

docker/image/Dockerfile

@@ -39,4 +39,4 @@ COPY requirements.txt constraints.txt .
 RUN . /opt/ros2doc/bin/activate && pip3 install -r requirements.txt -c constraints.txt
 
 RUN echo "source /opt/ros2doc/bin/activate" >> /home/$user/.bashrc
-USER $user
+USER $user