You might be able to restart the container with
docker start app
You might also be able to solve your problem with
apt install docker-ce docker-ce-cli
Not sure about the issue with the overlay. Have you done an os upgrade from an older Ubuntu at some point?
If you’re on digital ocean you can create a static ip pointing to the old server and update dns to point to it. Then when you move to the new server there is no dns lag since you can redirect the ip to the new server.