Add GitHub workflow

.github/workflows/check.yml

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+name: Check
+
+# Controls when the action will run. Triggers the workflow on push or pull request
+# events but only for the master branch
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+# A workflow run is made up of one or more jobs that can run sequentially or in parallel
+jobs:
+  # This workflow contains a single job called "build"
+  check:
+    # The type of runner that the job will run on
+    runs-on: ubuntu-latest
+
+    # Steps represent a sequence of tasks that will be executed as part of the job
+    steps:
+    # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
+    - uses: actions/checkout@v2
+
+    - name: Shell Linter
+      run: docker run -v $GITHUB_WORKSPACE:/mnt -w /mnt koalaman/shellcheck reStream.sh
+
+    - name: Shell Formatter
+      run: docker run -v $GITHUB_WORKSPACE:/mnt -w /mnt mvdan/shfmt -d reStream.sh