Relay unable to authenticate - upstream request returned error 503 Service Unavailable

### Version

21.8.0

### Steps to Reproduce

RockyLinux 8
Docker 20.10.8, build 3967b7d
Docker-Compose 1.29.2, build 5becea4c

standard install single org ** behind corporate proxy **

same issue as https://github.com/getsentry/onpremise/issues/966

### Expected Result

everything should work and should be able to send events to sentry

### Actual Result

Looking for help.

due to relay error "2021-08-27T17:27:51Z [relay_server::actors::upstream] ERROR: authentication encountered error: upstream request returned error 503 Service Unavailable caused by: no error details"

due to above error unable to send events to sentry

same as https://github.com/getsentry/onpremise/issues/966 tcpdump shows relay trying to connect to corporate proxy instead of web and kafka

Troubleshooting:
- same setup **not behind corporate proxy** everything works as expected and can send events to sentry.
- on setup behind corporate proxy tried manually configuring an internal network and assigning sentry containers to it and the nginx container to the internal and a bridge network - didn't work, probably overlooking something.
- on setup behind corporate proxy tried adding sentry network to the container ENV: no_proxy - didn't work still get relay error. got the idea from https://github.com/kubernetes-sigs/kind/issues/525

still trying a few things, will update if I find something that works. @semblano what worked for you, if you don't mind sharing?

would really like to see sentry network be internal after setup completes so that only nginx is exposed.