Pin GitHub Actions workflows to a specific version (#30)

Author: ErikSchierboom

.github/workflows/dockerhub.yml

@@ -10,12 +10,12 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5
 
-      - uses: satackey/action-docker-layer-caching@v0.0.5
+      - uses: satackey/action-docker-layer-caching@6b09a11416d285a6bf2a9d1ce2484c878f7c985e
 
       - name: Build and push Docker images
-        uses: docker/build-push-action@v1.1.0
+        uses: docker/build-push-action@92e71463491f2d026a477188b8ad3a0fdd9d672c
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_PASSWORD }}