support/docker: add flake8 command

When check-package fail due to flake8 coding style checks, it suggest
to use flake8 command line:

  run 'flake8' and fix the warnings

But flake8 is actually missing from the container used by
utils/docker-run, so add it to the container as well.

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/docker/Dockerfile

@@ -30,6 +30,7 @@ RUN apt-get -o APT::Retries=3 install -y --no-install-recommends \
         cpio \
         cvs \
         file \
+        flake8 \
         g++-multilib \
         git \
         libc6:i386 \