CI: add ci to lint our markdown

let's keep things consistent.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>

.ci/markdown.sh

+#!/bin/sh -e
+# Description: lint all markdown files
+# https://postmarketos.org/pmb-ci
+
+if [ "$(id -u)" = 0 ]; then
+	set -x
+	apk add npm
+	npm install -g markdownlint-cli
+	ln -sf /usr/local/lib/node_modules/markdownlint-cli/markdownlint.js /usr/bin/markdownlint-cli
+	exec su "${TESTUSER:-build}" -c "sh -e $0"
+fi
+
+MDL_FAILED=0
+find . -name '*.md' |
+while read -r file; do
+	echo "mdl: $file"
+	markdownlint-cli "$file" || MDL_FAILED=1
+done
+
+if [ "$MDL_FAILED" = "1" ]; then
+	echo "markdown lint failed!"
+	exit 1
+fi
+

.gitlab-ci.yml

+image: alpine:latest
+
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+
+default:
+  before_script:
+    - apk upgrade -U
+    - adduser -D build
+
+markdownlint:
+  script:
+    - ".ci/markdown.sh"
+