Add example to reproduce issue #8854: env var fallback in volume paths (#66)

Demonstrates that array/long syntax volumes with ${VAR:-default} fallback
values cause "Convert to file" to fail with a command injection error.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose-env-var-fallback-volume/default-config.yaml

@@ -0,0 +1,9 @@
+server {
+    listen 80;
+    server_name localhost;
+
+    location / {
+        root /usr/share/nginx/html;
+        index index.html;
+    }
+}