Improve dockerfile shutdown

By execing python3 rather than running it as a child of /bin/sh shutdown
becomes a lot faster because then the SIGTERM gets seen by the python
process; when /bin/sh is the CMD and is signalled it doesn't pass the
signal on to its child python process, and Docker then waits for 10
seconds before sending a SIGKILL.

Dockerfiles/ubuntu-20.04/focal-flask

@@ -35,4 +35,4 @@ RUN git archive --format=tar.gz -o /tmp/pl.tgz HEAD
 RUN tar -C /var/www/html -x -f /tmp/pl.tgz
 WORKDIR /code
 ENV CRSID=${CRSID}
-CMD python3 rundebug.py $CRSID
+CMD exec python3 rundebug.py $CRSID