Add shellcheck CI (MR 2)

.ci/shellcheck.sh

+#!/bin/sh -e
+# Description: lint all shell scripts
+# https://postmarktos.org/pmb-ci
+
+if [ "$(id -u)" = 0 ]; then
+	set -x
+	apk -q add shellcheck
+	exec su "${TESTUSER:-build}" -c "sh -e $0"
+fi
+
+find . -name '*.sh' |
+while read -r file; do
+	echo "shellcheck: $file"
+	shellcheck "$file"
+done

.gitlab-ci.yml

+# global settings
+image: alpine:latest
+stages:
+  - lint
+
+# shellcheck
+shellcheck:
+  stage: lint
+  image: alpine:edge
+  before_script:
+    - adduser -D build
+  script:
+    - .ci/shellcheck.sh