I prefer alcohol to sweets, so I'm definitely a drinker.
彼は辛党だから、デザートよりビールを選ぶ。
He's a drinker, so he'll choose beer over dessert.
person who likes spicy food
Colloquial extension of the original meaning. Uses the kanji 辛 (spicy) literally, unlike the alcohol sense.
辛党の友達と激辛ラーメンを食べに行った。
I went to eat super-spicy ramen with a friend who loves spicy food.
My father is a drinker who likes alcohol more than sweets.
He's a serious drinker who has alcohol every night.