HADOOP-16849. start-build-env.sh behaves incorrectly when username is numeric only. Contributed by Jihyun Cho.

(cherry picked from commit 9709afe67d8ed45c3dfb53e45fe1efdc0814ac6c)

Conflicts:
	start-build-env.sh

start-build-env.sh

@@ -46,5 +46,5 @@ docker run --rm=true -t -i \
   -v "${PWD}:/home/${USER_NAME}/hadoop" \
   -w "/home/${USER_NAME}/hadoop" \
   -v "${HOME}/.m2:/home/${USER_NAME}/.m2" \
-  -u "${USER_NAME}" \
+  -u "${USER_ID}" \
   "hadoop-build-${USER_ID}"