restore comment as fixing it is unrelated to the migration

linux_docker_resources/Dockerfile

@@ -100,6 +100,10 @@ RUN if test ${INSTALL_TURTLEBOT2_DEMO_DEPS} = true; then apt-get update && apt-g
 # https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=819741
 # https://bugs.launchpad.net/ubuntu/+source/vtk6/+bug/1573234
 # https://github.com/PointCloudLibrary/pcl/issues/1594
+# TODO(clalancette): for some reason, if these two workarounds are listed in
+# different RUN statements, it causes the docker image to fail to build on
+# aarch64 (see https://github.com/ros2/ci/pull/73).  As a workaround, do them
+# in one statement.
 RUN if test ${INSTALL_TURTLEBOT2_DEMO_DEPS} = true; then sed -i -e '/vtkproj4;/d' /usr/lib/$(uname -m)-linux-gnu/cmake/pcl/PCLConfig.cmake && ln -s /usr/lib/$(uname -m)-linux-gnu/libproj.so.9.1.0 /usr/lib/$(uname -m)-linux-gnu/libproj.so; fi
 
 # Create a user to own the build output.