make all UserAgent checks case-insensitive (#4)

* Handle alternate case for docker and Go user-agents

Some docker instances are unable to login through Beyond. That's because the user-agent is spelled with a D instead of a d.
Examples of these user-agents include:
- Docker-Client/20.10.7 (darwin)
- Docker-Client/nerdctl-v0.12.1 (linux)

* Update to use ToLower

Co-authored-by: LeoChen <[email protected]>

docker.go

@@ -94,9 +94,9 @@ func (ds *dockerServer) ModifyResponse(resp *http.Response) error {
 
 func (ds *dockerServer) RegisterHandlers(mux *http.ServeMux) {
 	mux.HandleFunc(ds.host+"/", func(rw http.ResponseWriter, r *http.Request) {
-		ua := r.UserAgent()
+		ua := strings.ToLower(r.UserAgent())
 		ua1 := strings.HasPrefix(ua, "docker/")
-		ua2 := strings.HasPrefix(ua, "Go-")
+		ua2 := strings.HasPrefix(ua, "go-")
 		if !ua1 && !ua2 {
 			handler(rw, r)
 			return