add github actions workflow for shellcheck (#7)

Author: anlutro

.github/workflows/run-tests.yml

@@ -0,0 +1,18 @@
+name: run-tests
+
+'on':
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - '**'
+
+jobs:
+  shellcheck:
+    runs-on: ubuntu-latest
+    container:
+      image: koalaman/shellcheck-alpine:stable
+    steps:
+      - uses: actions/checkout@v2
+      - run: ./shellcheck.sh