Add GithHub Dependabot for Actions

And specify exact versions

Author: kare

.github/dependabot.yaml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"

.github/workflows/test.yaml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     name: Testing streamr-docker-dev-action
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v2.3.4
     - uses: streamr-dev/streamr-docker-dev-action@master
     - run: curl -v http://localhost/api/v1/products
       shell: bash