ECS: Should I use ALB healthchecks, container healthchecks, or both?

The status is UNKNOWN because the ECS health check is not defined.
It is UNKNOWN as per specifications.
https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_HealthCheck.html

If you are using ALB health checks, there is no need to add ECS health checks as well.
If the health check path accessed by the ALB health check check checks the normality of the application, then there should be no problem.
If ALB is slow to become healthy, it may be a good idea to review the health check thresholds, etc.

Incidentally, if both health checks are set, the task will be replaced if one of them becomes UNHEALTHY.

I've just been asking myself the same question.

I'm sure the previous answers are correct that there is no strict requirement to add ECS health checks, but I absolutely hate that the ECS console is going to report a health status of UNKNOWN forever if I just rely on the ALB.

Therefore I am planning to add the ECS health checks anyway.

Would be great if AWS could make this a bit more intuitive, maybe add a status of NOT REQUIRED if the health checks are really not required.

No need of setting up health checks for ecs container if ALB health check is already in place. Incase your ALB health check is slow then revisit your health check setting in the target group attached with your ALB and look for below.

1. Healthy threshold
2. Path