docs: clarify ECR_REGISTRY as optional override in env.example

Author: drernie

env.example

@@ -24,8 +24,8 @@ FASTMCP_TRANSPORT=stdio
 NGROK_DOMAIN=my-free-domain.ngrok-free.app
 
 # Docker Configuration
-# ECR registry URL (required for Docker push operations)
-# Format: {account_id}.dkr.ecr.{region}.amazonaws.com
+# ECR registry URL (optional override for Docker push operations)
+# Default: {account_id}.dkr.ecr.{region}.amazonaws.com
 ECR_REGISTRY=123456789.dkr.ecr.us-east-1.amazonaws.com
 
 # Docker image configuration (optional)