HADOOP-16849. start-build-env.sh behaves incorrectly when username is…

… numeric only. Contributed by Jihyun Cho.

(cherry picked from commit 9709afe)

 Conflicts:
	start-build-env.sh

start-build-env.sh

@@ -78,5 +78,5 @@ docker run --rm=true $DOCKER_INTERACTIVE_RUN \
   -v "${PWD}:/home/${USER_NAME}/hadoop${V_OPTS:-}" \
   -w "/home/${USER_NAME}/hadoop" \
   -v "${HOME}/.m2:/home/${USER_NAME}/.m2${V_OPTS:-}" \
-  -u "${USER_NAME}" \
+  -u "${USER_ID}" \
   "hadoop-build-${USER_ID}" "$@"