fix: guard against invalid proxy values due to move of port mapping to ports plugin

Users somewhat infrequently set a port mapping as the proxy type, causing issues in exposing apps publicly. Ideally we list out the available proxy types, but that would be a potential bc-break for non-core plugins, so this is the way to go for now.

Closes #6764

plugins/proxy/subcommands.go

@@ -2,6 +2,7 @@ package proxy
 
 import (
 	"errors"
+	"strings"
 
 	"github.com/dokku/dokku/plugins/common"
 	"github.com/dokku/dokku/plugins/config"
@@ -89,6 +90,11 @@ func CommandSet(appName string, proxyType string) error {
 		return errors.New("Please specify a proxy type")
 	}
 
+	if strings.Contains(proxyType, ":") {
+		common.LogWarn("Detected potential port mapping instead of proxy type")
+		return errors.New("Consider using ports:set command or specifying a valid proxy")
+	}
+
 	key := "DOKKU_APP_PROXY_TYPE"
 	if appName == "--global" {
 		key = "DOKKU_PROXY_TYPE"