Adds new environment variables that can be used with the Docker image
(`SYNAPSE_HTTP_PROXY`/`SYNAPSE_HTTPS_PROXY`/`SYNAPSE_NO_PROXY`)

Useful for things like the [Secure Border
Gateway](https://element.io/server-suite/secure-border-gateways)


### Why is this necessary?

You can already configure the `HTTP_PROXY`/`HTTPS_PROXY` environment
variables to proxy outbound requests but setting this globally in the
Docker image affects all processes which isn't always desirable or
workable in the case where the proxy is running in the Docker image
itself (because the Debian packages will fail to download because the
proxy isn't up and running yet) . Adding Synapse specific environment
variables
(`SYNAPSE_HTTP_PROXY`/`SYNAPSE_HTTPS_PROXY`/`SYNAPSE_NO_PROXY`) makes
things much more targetable.