Enable dualstack in every net.Dialer, fixes #9364

Default transport enables it:

* https://github.com/golang/go/blob/d2826d3e06/src/net/http/transport.go#L42-L46

```
    DialContext: (&net.Dialer{
	Timeout:   30 * time.Second,
	KeepAlive: 30 * time.Second,
	DualStack: true,
    }).DialContext,
```

See also: https://github.com/golang/go/issues/15324

pkg/components/imguploader/webdavuploader.go

@@ -23,7 +23,8 @@ type WebdavUploader struct {
 var netTransport = &http.Transport{
 	Proxy: http.ProxyFromEnvironment,
 	Dial: (&net.Dialer{
-		Timeout: 60 * time.Second,
+		Timeout:   60 * time.Second,
+		DualStack: true,
 	}).Dial,
 	TLSHandshakeTimeout: 5 * time.Second,
 }