server: fix Transport override (#7834)

This changes makeRequest to update the http client Transport if and only
if testMakeRequestDialContext is set. This is to avoid overriding the
default Transport when testMakeRequestDialContext is nil, which broke
existing behavior, included proxies, timeouts, and other behaviors.

Fixes #7829
Fixes #7788

server/images.go

@@ -1076,17 +1076,15 @@ func makeRequest(ctx context.Context, method string, requestURL *url.URL, header
 		req.ContentLength = contentLength
 	}
 
-	resp, err := (&http.Client{
-		Transport: &http.Transport{
-			DialContext: testMakeRequestDialContext,
-		},
+	c := &http.Client{
 		CheckRedirect: regOpts.CheckRedirect,
-	}).Do(req)
-	if err != nil {
-		return nil, err
 	}
-
-	return resp, nil
+	if testMakeRequestDialContext != nil {
+		tr := http.DefaultTransport.(*http.Transport).Clone()
+		tr.DialContext = testMakeRequestDialContext
+		c.Transport = tr
+	}
+	return c.Do(req)
 }
 
 func getValue(header, key string) string {