Add CI workflows for dual-image channel pattern

- Non-prod workflow: builds on push to main, pushes to acme-nonprod registry, creates Octopus release in Non-Prod channel
- Prod workflow: builds on v* tags, pushes to acme-prod registry, creates Octopus release in Prod channel
- Simple Alpine Dockerfile for demo purposes

README.md

@@ -1,3 +1,16 @@
-# demo-app
+# Demo App — Dual Image Channel Pattern
 
-Demo app for dual-image channel pattern
+Demonstrates Octopus Deploy's channel pattern for routing non-prod and prod container images through separate lifecycles.
+
+## How it works
+
+- **Push to `main`** → builds non-prod image → pushes to `acme-nonprod/demo-app` → creates Octopus release in Non-Prod channel
+- **Push a `v*` tag** → builds prod image → pushes to `acme-prod/demo-app` → creates Octopus release in Prod channel
+
+Each channel has its own lifecycle:
+- Non-Prod: Development → Staging (cannot reach Production)
+- Prod: Staging → Production
+
+## Octopus Space
+
+https://taniwha.octopus.app/app#/Spaces-102