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/docker-compose.yaml

@@ -0,0 +1,8 @@
+services:
+  web:
+    image: nginx:alpine
+    volumes:
+      - type: bind
+        source: '${CONFIG_FILE:-./default-config.yaml}'
+        target: /etc/nginx/conf.d/custom.conf
+        read_only: true