support/docker: add shellcheck

Allow developers to run check-package for init scripts, that call
shellcheck, without having to install the tool.

Since the docker have a fixed version of the tool, there will be no
difference between runs in different machines.

One can call:
$ utils/docker-run utils/check-package package/package/S*
$ utils/docker-run shellcheck package/package/S*

This change also allows to eventually run check-package for init scripts
in the GitLab CI.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Acked-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

support/docker/Dockerfile

@@ -45,6 +45,7 @@ RUN apt-get install -y --no-install-recommends \
         qemu-system-arm \
         qemu-system-x86 \
         rsync \
+        shellcheck \
         subversion \
         unzip \
         wget \