Docker Sandbox: Extract request proxying into swappable containers. #901
Description
From the docs, it sounds like Docker Sandbox automatically proxies some external requests through some built-in proxy server (which deals with API key management). This is a really useful feature, but the current design is antithetical to Docker's normal system of leaving such concerns up to the user or a container. I do like the feature that makes it easy to proxy external requests and this is very useful for many projects, but the proxy server should be supplied in the form of a docker image and not built in to the system.
Instead of the current proxy, the user should be able to provide their own proxy container as long as it matches some public facing surface area. The user could then do anything they want inside of that proxy, such as adding arbitrary headers to outgoing requests, filtering/modifying responses, etc. This would make the system work with any arbitrary future APIs rather than just the blessed set that the Docker team has chosen to support (or been paid to support).