correcting some behaviors for $DOCKER_HOST, now uses container's private IP instead of localhost

internal/proxy/fields/host.go

@@ -4,8 +4,10 @@ import (
 	E "github.com/yusing/go-proxy/internal/error"
 )
 
-type Host string
-type Subdomain = Alias
+type (
+	Host      string
+	Subdomain = Alias
+)
 
 func ValidateHost[String ~string](s String) (Host, E.NestedError) {
 	return Host(s), nil