ci: add shellcheck (bash linter)

.gitlab-ci.yml

+---
+# -*- coding: utf-8 -*-
+#
+# My GiLab Runner (mygr) is a script for loaunching local GitLab Runner on workstation
+# Copyright (C) 2020  Christophe Chaudier
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+include:
+  - https://gitlab.com/lydra/gitlab-ci-templates/-/raw/master/gci-templates/activate-dind.yml
+  - https://gitlab.com/lydra/gitlab-ci-templates/-/raw/master/gci-templates/stages.yml
+  - https://gitlab.com/lydra/gitlab-ci-templates/-/raw/master/gci-templates/job/bash-lint.yml