Docker fails to start on Ubuntu 15.04

I installed Docker on Ubuntu 15.04 using:

wget -qO- | sh

as described at Install Docker on Ubuntu.

I added myself to the docker group:

sudo usermod -aG docker balaaman

Then I logged out and logged in again, and ran:

docker run hello-world

and saw this:

Cannot connect to the Docker daemon. Is 'docker -d' running on this host?

When I tried to start the Docker daemon like this:

sudo service docker start

I got this error:

Failed to start docker.service: Unit docker.service is masked.

Thanks to Yannick Lizzi on this thread I found this fix:

systemctl unmask docker.service
systemctl unmask docker.socket
systemctl start docker.service

After this, I logged out and logged in again (again) and “docker run hello-world” worked fine, and all was well.