CI: Add .gitlab-ci.yml

This gives us neat little CI integration. No more "you have a
different version of shellcheck" - we just pick the one everyone can
run with docker. Also, no more "oops, forgot to run make check", that
is, if we adopt a merge request based flow.

Even without this this is useful, since if you do a fork and test
there, gitlab CI will run stuff for you. Example:

https://gitlab.freedesktop.org/danvet/maintainer-tools/blob/master/.gitlab-ci.yml



Observe the awesome green checkmark in the top-left corner!

v2: Use python3 (Lucas). And fix whitespace.

v3: Go back to python2, on fedora the python3 packages have a -3
suffix. Don't ask.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

.gitlab-ci.yml

+image: fedora:latest
+
+before_script:
+  - dnf install -y ShellCheck python2-docutils python2-sphinx make
+
+check:
+    stage: build
+    script:
+      - make check