It appears that milk was delivered to the dormitories first thing in the morning, after the same fashion as "the milkman". Distributing said milk was a separate duty from waking up the students, but occurred around the same time; both duties being on a weekly rotation. Therefore it was mutually agreed that the student on milk duty would also bang on the doors as he went, thus relieving the student on Official Door Banging Duty that week from having to get up early to do it. It's unclear why this is described as as a "lie-in", as one would still presumably get woken up along with everyone else...