If you are travelling United in International First Class you will (even without Star Alliance elite status) have access to the LH Lounges (there are two - a Business Class Lounge and a LH Senator Lounge). These two lounges are located in the B Concourse (with the Senator Lounge being the nicer of the two). They do have showers. However, as pointed out by another post, they are nowhere near where your UA flight will depart and it would take a bit of time to get there, use the showers and then get to your UA gate in time. Not worth it IMO.