fix: strip /github prefix in auth proxy github case

The public /github/ route uses http.StripPrefix to remove /github
before forwarding to githubProxy. The auth proxy's github case was
missing this strip, causing /arkylin to be forwarded as
/github/arkylin → 404 on GitHub.

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <[email protected]>

cmd/main.go

@@ -178,10 +178,7 @@ func buildHandler(cfg *config.Config) (*admin.Handler, http.Handler) {
 			}
 			ghcrProxy.ServeHTTP(w, r)
 		case "github":
-			if !strings.HasPrefix(r.URL.Path, "/github/") {
+			middleware.StripPrefix("/github", githubProxy).ServeHTTP(w, r)
-				r.URL.Path = "/github" + r.URL.Path
-			}
-			githubProxy.ServeHTTP(w, r)
 		default:
 			dockerProxy.ServeHTTP(w, r)
 		}