Add shellcheck linter workflow

Fixes #57

.github/workflows/linter.yml

@@ -0,0 +1,27 @@
+name: Lint Code Base
+
+on:
+  push:
+    branches: master
+    paths:
+      - '**.sh'
+  pull_request:
+    branches: master
+    paths:
+      - '**.sh'
+jobs:
+  build:
+    name: Lint Code Base
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v2
+        with:
+          # Full git history is needed to get a proper list of changed files within `super-linter`
+          fetch-depth: 0
+      - name: Lint Code Base
+        uses: github/super-linter@v4
+        env:
+          VALIDATE_ALL_CODEBASE: true
+          DEFAULT_BRANCH: master
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Allow extra arguments to restic-backup with `$RESTIC_BACKUP_EXTRA_ARGS`.
 - Add `$RESTIC_VERBOSITY_LEVEL` for debugging.
 - Assertion on all needed envionment variables in the backup and check scripts.
+- Added linter (`shellcheck(1)`) that is run on push and PRs.
 
 ### Changed
 - **BREAKING CHANGE** renamed