add hello-world action

as per https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-docker-container-action

action.yml

@@ -0,0 +1,16 @@
+# action.yml
+name: "Hello World"
+description: "Greet someone and record the time"
+inputs:
+  who-to-greet: # id of input
+    description: "Who to greet"
+    required: true
+    default: "World"
+outputs:
+  time: # id of output
+    description: "The time we greeted you"
+runs:
+  using: "docker"
+  image: "Dockerfile"
+  args:
+    - ${{ inputs.who-to-greet }}