Make docker build work without yarn.lock file

Test plan
- dc build or docker build . work in the
  packages/canvas-planner directory

Change-Id: Id66b9b7e608f0dff4e416a94e3d839211d190620
Reviewed-on: https://gerrit.instructure.com/152583
Tested-by: Jenkins
Reviewed-by: Jon Willesen <jonw+gerrit@instructure.com>
Product-Review: Mysti Sadler <mysti@instructure.com>
QA-Review: Mysti Sadler <mysti@instructure.com>

packages/canvas-planner/Dockerfile

@@ -17,7 +17,6 @@ USER root
 RUN mkdir -p $APP_HOME
 
 COPY package.json $APP_HOME/
-COPY yarn.lock $APP_HOME/
 
 WORKDIR $APP_HOME