it's the gate right behind you at the fountain when you are talking to the hologram of Elijah. Right where you start the dlc. It's also the gate you leave by so you can load up over-encumbered and not have a long slodge to the exit point.
-Gunny out.
@ the OP:
After you first talk to Elijah, turn around and head for the gate. When you get close he'll mention that it's the way out and locked for the time being. Turn left and look carefully and there will be a half-buried, beaten-to-hell-looking trash can that is perfectly safe to store stuff in.
For that matter,
every container not in the vault is safe to store in, although the ones in the other sectors are not exactly convenient for extended use.