[FEATURE]: drizzle-gateway - Support configuring the server listen address for IPv6 private networks

[howyi]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I am running Drizzle Gateway behind Cloudflare Tunnel on Railway, using both official templates:

• Drizzle Gateway for Railway
• Cloudflare Tunnel for Railway

The tunnel connects to:

http://drizzle-gateway.railway.internal:4983

However, cloudflared cannot connect:

dial tcp [fd12:...]:4983: connect: connection refused

Railway recommends listening on :: for IPv6 private networking.

The published Gateway image (ghcr.io/drizzle-team/gateway:1.4.0) appears to bind to 0.0.0.0 and only reads the PORT environment variable. Therefore, setting HOST=:: and redeploying had no effect.

Could Gateway support a HOST or HOSTNAME environment variable, or listen on :: by default?

Using a public Railway URL as the tunnel origin is undesirable because it creates another public path that may bypass Cloudflare Access.