environment(docker): set outgoing ip correctly (#135)

Closes https://github.com/pterodactyl/panel/issues/3841

environment/allocations.go

@@ -12,6 +12,11 @@ import (
 // Defines the allocations available for a given server. When using the Docker environment
 // driver these correspond to mappings for the container that allow external connections.
 type Allocations struct {
+	// ForceOutgoingIP causes a dedicated bridge network to be created for the
+	// server with a special option, causing Docker to SNAT outgoing traffic to
+	// the DefaultMapping's IP. This is important to servers which rely on external
+	// services that check the IP of the server (Source Engine servers, for example).
+	ForceOutgoingIP bool `json:"force_outgoing_ip"`
 	// Defines the default allocation that should be used for this server. This is
 	// what will be used for {SERVER_IP} and {SERVER_PORT} when modifying configuration
 	// files or the startup arguments for a server.