I installed Docker on Ubuntu 15.04 using:
wget -qO- https://get.docker.com/ | 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.