It's incredibly hard to say there's one thing every goaltender should do before a skate, because every body is different. Even so, 'what should every goalie do?' is something Dan gets asked almost every day. He shares one easy exercise that fits into your pre-skate prep as well as some general warm-up tips in a post on TheGoalNet here.
If you want to skip the article and go straight to his pre-skate hip mobility exercise, here you go: https://www.winyourwarmup.com/90-90hipliftwinternalrotation