chore: Add alpine-base example for multi-platform testing

- Add example/alpine-base project that uses Alpine Linux as base image
- Update CI extended-platforms job to use the example directory
- Simplifies CI configuration by moving test project to examples

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

example/alpine-base/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "test-alpine"
+version = "0.1.0"
+edition = "2021"
+
+[package.metadata.krust]
+base-image = "alpine:latest"

example/alpine-base/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello from Alpine multi-platform test!");
+}