Last time I checked ECS was even more expensive than using Lambda but without the ability of fast starting your container, so I really don't get the niche it fits into, compared to Lambda on one side and self-hosting docker on minimal EC2 instances on the other side.

I may need to look at Lambda closer! At least way back, I thought it was a no-go since the main runtime I work with is Ruby. As for minimal EC2 instances, definitely, I do that for environments where it makes sense and that's the case fairly often.