Undertow in 7.1.0.DR9 doesn't serve welcome-content after the ports 8080 and 9990 are open. It rather waits for full server boot and returns status "503 Service Unavailable" in the meantime.
The welcome page was returned in DR8 and earlier versions. We use this approach in Docker image tests - to see if the EAP container starts correctly.
The change is probably caused by EAP7-636 - server start in suspended mode. If the 503 is the correct behavior, we should at least give users information "booting, retry later". Current behavior doesn't allow to find what's wrong with the server. (Is it an internal error? Is it just temporary? Should be the request retried later?).
We could use some HTTP header to report server status during 503 ("booting/shutting down/..."). If it's booting we could also add HTTP header Retry-After with some reasonable/configurable default.