First, thanks for your amazing work on localstack!

Please treat this as a start of a conversation rather than a full-featured PR. I'm not certain that this is the right direction and would love your feedback.

Motivation

The issue I'm having is that I'm running localstack behind a proxy, which is running on a specific host (which is covered by HOSTNAME_EXTERNAL, but is also on a different port from that of the proxy. So, when SQS returns the queue URL it returns the HOSTNAME_EXTERNAL that I set, but returns the local port and not the port that the client requests the proxy with.

[proxy:80] -> [localstack:4576] -> [elasticmq:4561]

What I've done at the moment, is I've set my SQS port to :80 (which is what my proxy wants). This works, but in my opinion is more of a workaround because any other service needs to return a URL pointing back to itself, I will not be able to also put it on :80 as it will be taken by SQS.

Proposed solution

So, the solution I'm proposing is to define a <SERVICE>_PORT_EXTERNAL and use that along with HOSTNAME_EXTERNAL. Alternatively (and if there are no plans to use HOSTNAME_EXTERNAL as a single external hostname for all services) we can change it to something like <SERVICE>_EXTERNAL_URI, which would be a host:port combination specific to a service.