Today I was creating AWS EC2 instances using docker-machine but if the instances where in a custom VPC the creation process got stuck waiting for sshd server to be up.
I then tried to create these instances manually but the problem persisted. I could not ssh to the public ip after the instance creation.
Error was simple: The custom VPC did not have a gateway to connect it to the internet. To solve it was only necessary to create the Gateway, associate it with the VPC and create a new routing entry that made all remaining traffic (0.0.0.0) to be forwarded to the gateway.