Ah, we don’t do normal HTTP caching (like from volumes). The [statics] are designed specifically for assets that are bundled as part of a full stack app. When you deploy an app with statics, we extract the files from the Docker image itself and handle them separately.
We’ll probably need to have HTTP caching-like-a-CDN someday. For now, the best way to solve this is to either deploy an nginx app that does caching, or put a traditional CDN in front of your app.