Add github workflow

.github/actions/attic-cache/action.yml

@@ -0,0 +1,15 @@
+name: "Attic Watch and Push"
+description: "Composite action pushes results after the job completes. Needs prior attic-cache action to have executed successfully."
+
+inputs:
+  cache_repository:
+    description: "Cache repository to use with attic"
+    required: true
+
+runs:
+  using: "composite"
+  steps:
+    - name: Attic Push Results
+      shell: "nix shell nixpkgs#attic-client --command {0}"
+      run: |
+        attic push "${{ inputs.cache_repository }}" "./result" # TODO: Make this smarter