My mongo instance grew up to the RAM limit I've set to its container. What happens if it eventually reaches the limit? Crash? Performance decline? Or is it just dropping stale indexes?
The memory namespace your mongo instance runs in will run out of memory, after which the normal linux OOM behaviour applies, that is the kernel will kill mongod if there are no other processes. If you do not want this to happen, you need to limit MongoDBs memory usage to a small bit below the hard limit of your container.