What's the AD rammus dominion build?
Really depends on the situation. I tend to get Berserker's Greaves, Phantom Dancer, and Bloodthirster with AD Rammus as my "core" items. Any other AD carry items tend to work well (Infinity Edge, Black Cleaver, Last Whisper, a second Phantom Dancers, etc.). Buying health is never a bad idea either. I used to get Warmogs for sure with AD Rammus in Dominion until they removed it. I generally don't recommend many tanky items with AD Rammus for Dominion. You should be relying on his shield for defense, as you'll be able to take out most champions 1v1 before your shield runs out (assuming you taunt them and chase them down when they run away!).
He has a weak start and is very susceptible to dying early game. However, thanks to his speed from his Q and the fast-paced nature of Dominion, this is not so much of an issue. If you focus on capturing and defending early game while avoiding deaths, you'll have enough items to be a formidable fighter fairly quickly. AD Rammus shines late game in Dominion and is probably one of the best champions at turning around a Dominion match. Don't let his weak start discourage you!
Also keep in mind that armor translates into additional AD for Rammus. Sometimes it is not a bad idea to buy armor (Thornmail is great against AD teams). This also means that it is more important to focus on attack speed than pure attack damage, as you'll do most of your fighting while using your W shield (boosts his armor, which boosts his attack).
Because of his Q, don't forget to try and capture points that the enemy is not paying attention to. You can get in and out of almost any situation very quickly. He's great at capturing points quickly or pulling the enemy away from points they are defending/attacking so that your teammates have an easier time.
Never be afraid to defend, either. If you have 3 points and can't easily take more without losing what you have, just turtle your points. I generally pick Garrison as one of my summoner spells for this purpose (or capturing points). AD Rammus can often defend a turret against 3 enemies (sometimes more) by himself. Pop Garrison when they move in to attack, taunt your primary target, pop your shield, pop your ult, and lay into them with auto attacks while keeping in the boundaries of the capture point.
I tend to pick standard AD masteries and runes with him as well in these situations, though it really doesn't hurt if you want to use those to give him a slight tanky edge.