CI: add yamllint to CI pipeline

Signed-off-by: Jan-Tarek Butt <tarek@ring0.de>

.ci/yaml_rules.yaml

+---
+
+extends: default
+
+rules:
+  line-length:
+    level: error
+    max: 100
+    allow-non-breakable-inline-mappings: true

.ci/yamllint.sh

+#!/bin/sh
+
+echo "Doing yamllint..."
+ret=0
+# explicitly set IFS to contain only a line feed
+IFS='
+'
+
+filelist="$(find . -not \( -path './.git/*' -prune \) -type f ! -name "$(printf '*\n*')")"
+for line in $filelist; do
+  if echo "$line" | grep -q -E '.*\.(yml|yaml)' ; then
+    if ! grep -q "$line" ".ci/shell_accepted"; then
+      tmp="$(yamllint --strict -d ./.ci/yaml_rules.yaml "$line")"
+      if ! [ "$tmp" = "" ]; then
+	echo "$tmp"
+        ret=1
+      fi
+    fi
+  fi
+done
+exit $ret

.gitlab-ci.yml

 stages:
+  - lint
   - build
   - deploy
   - production
 
+lint:
+  stage: lint
+  image:
+    name: cytopia/yamllint
+  script:
+    - ./.ci/yamllint.sh
+
 build:
   stage: build
   image: