Was thinking of using an old folgers coffee contai...

Unused coffee grounds are not optimal to throw in. Can cause ph swings as others has stated. However, used coffee grounds are perfectly OK. I throw all my coffee grounds in my compost, then my compost goes into weed garden. Happy growing.

it would impact pH too.... hopefully you'd just knock those crumbs out first...

Na, Just rinse it out. That’s hardly enough to cause any noticeable difference, but could throw off your pH or other stuff. I have used this exact container before tho.. just rinsed out well with some holes drilled in bottom for irrigation and little oxygen to the root zone

Coffee grounds will contain nitrogen, calcium, potassium, iron, phosphorus, and magnesium so it's not a BAD thing - but if you're also using nutes, you may be giving too much of some of the elements... Outside, putting coffee grounds around a plant will help attract worms and it's good for adding acid around acid-loving plants such as hydrangea - but I honestly think it would have limited worth for an indoor grow of cannabis... That being said, it's your call since I really don't see a HUGE downside.. Good luck!