[Docker vs. outbound traffic] How to wrap all outbound traffic in a specific container through a proxy?

Question

proxy

All the best!
Can you please tell me how to wrap all outgoing traffic from one container through a proxy? So that the external IP is different from the host's external IP?
Tried specifying env http_proxy and etc. but this works for wget & curl from command line. However, it doesn't work when executing a php script with file_get_contents
Thanks!