That tank was provided by the Swiss, to ensure that this particularly infamous El Al Pilot (notorious for missing his slot and delays) would notice the friendly reminder outside and taxi to the runway in time. It was also meant as a "follow me" vehicle, following a few incidents of the same pilot taking the wrong turn.
This practice is being used increasingly for other airlines as well, especially at important overcrowded hub airports. Heathrow is planning to use tanks to remind Lufthansa, Virgin Express and Aer Lingus pilots that the British regulations allow A320s and 737s to depart 1 minute after a 757. To make sure the pilot does not sit idle on the runway for 2 minutes, the gun is pointed at the plane if the engines do not powerup within 20 seconds of aligning with the runway. There are rumours that it might also be used to encourage Eastern European, Italian and Spanish pilots (and generally any pilots from less busy airfields) to speed up the ligning-up process a bit. Frankfurt is, of course, returning the favour by aiming the gun at British A320s should they wish to follow a 757 with less than 3 minutes clearance. At the request of Lufthansa, any Ryanair plane to be seen in the vicinity of Frankfurt Main is to be shot on sight. Similar procedures are being adapted for Easyjet planes that might fly above Heathrow, Ryanair planes seen at any popular passenger airport served by any major airlines, and the Pokemon plane at any European airport.
Hope this information helps you.