Support for global container environment variables

[vidister]

We have a customer that needs to route external traffic through a HTTP proxy. For this we set the environment variables HTTP_PROXY, HTTPS_PROXY and NO_PROXY in the containers.

Are you willing to merge a PR that implements generic global environment variables, that get passed into every container?

[leoluk]

Sounds good to me!

I'd prefer a generic "HTTP proxy" setting vs. a generic env mechanism just in case we add new services that accept proxy settings in a different way.