gitlab-ci/containers: Add missing wildcard where we should look for changes

The tests/docker/* wildcard seems to only match the files that are directly
in the tests/docker folder - but changes to the files in the directory
tests/docker/dockerfiles are currently ignored. Seems like we need a
separate entry to match the files in that folder. With this wildcard added,
the stages now get re-run successfully when something in the dockerfiles
has been changed.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200713182235.30379-1-thuth@redhat.com>