Add Watchtower trigger action definition.

action.yml

@@ -0,0 +1,26 @@
+name: 'Trigger Watchtower'
+author: 'Bas Weelinck'
+description: |
+  Wake up Watchtower by web request, so it will deploy any new images.
+
+inputs:
+  watchtower_url:
+      description: 'URL where Watchtower is listening'
+  watchtower_token:
+      description: 'Bearer token for authenticating to Watchtower'
+
+runs:
+  using: 'composite'
+  steps:
+    - name: Signal Watchtower
+      shell: bash
+      run: |
+        set -e
+        HTTP_CODE=$(curl -H "Authorization: Bearer ${{ inputs.watchtower_token }}" ${{ inputs.watchtower_url}}/v1/update -w '%{http_code}' -o /dev/null -s)
+        if [[ $HTTP_CODE == "200" ]]
+        then
+          echo 'Success!'
+        else
+          echo "Unexpected HTTP response code $HTTP_CODE"
+          exit 1
+        fi