How Do You Fix “Failed to start docker.service. Unit docker.socket is masked”?

Problem scenario
You try to start the Docker service (e.g., with sudo systemctl start docker). You get this error: "Failed to start docker.service. Unit docker.socket is masked"

What should you do?

Possible solution #1

sudo systemctl unmask docker.socket
sudo systemctl unmask docker.service

This solution was adapted from this blog.

Possible solution #2
Uninstall Docker. Install Docker via the docker-ce repo. If you install a different version of Docker, you may be able to get around this error.