Why am I getting "Cannot connect to the Docker daemon" when the daemon is running?
This question has already been answered, but here's an additional piece of information.
No matter if you're on Arch or another distribution like Fedora or Ubuntu, Docker uses a socket file to communicate. When you run docker commands, it uses this socket to talk to the Docker daemon. Of course, the daemon must be running (and it's often disabled by default), but if your user can't access the socket, it wouldn't be able to communicate with the daemon either.
You would first install Docker from the distribution's repository. Some people download an install script and pipe it to a shell (curl ... | sh), but it's recommended to install it from the repository so it can be updated easily.
Arch:
# pacman -S docker
Fedora:
# dnf install docker
As mentioned above, the daemon may be disabled by default. If you want to use Docker, the daemon must be running.
Enable it (so it will be started on boot):
# systemctl enable docker
Start it now (or reboot):
# systemctl start docker
Now, by default (if the docker group is missing), the Docker socket is owned by root:
# ls -la /var/run/docker.sock
srw-rw---- 1 root root 0 Apr 28 17:22 /var/run/docker.sock
This is why a regular user is not able to talk to the docker daemon. A regular user does not have sufficient permissions to access the socket. It's not able to reach the daemon, so it assumes it's not running and shows this error: Cannot connect to the Docker daemon. Is the docker daemon running on this host?
This is why many people simply start all Docker commands as root, using sudo. But as described in the other answer, Docker has its own mechanism for that, so using sudo is not necessary.
Ideally, a group called docker is created when installing Docker. However, if that group does not exist when the daemon is started, the socket file is owned by root.
In some cases, that group used to have a different name, like dockerroot on Fedora. Check grep docker /etc/group to see if there is such a group on your system. If you're already using that group (your user is in it), you would need to configure Docker to use it:
In /etc/sysconfig/docker, add -G dockerroot (note: it's a workaround, not the best solution):
OPTIONS='--selinux-enabled -G dockerroot'
After restarting the daemon, your user will be able to access the socket:
# systemctl restart docker
# ls -la /var/run/docker.sock
srw-rw---- 1 root dockerroot 0 Apr 28 17:32 /var/run/docker.sock
Otherwise, the official way would be to use the group called docker. If it exists, Docker will automatically use it, i.e., set the socket's group to that group. If it doesn't exist, all you need to do is create it and restart the daemon:
# groupadd docker
# systemctl restart docker
The socket file will be owned by that group:
# srw-rw---- 1 root docker 0 Apr 28 17:42 /var/run/docker.sock
Your user must be in the docker group to be able to access the socket:
# usermod -aG docker (user)
You may have to log out and log back in again (or su - (user)), run id to see if you're in the group.
You can then use Docker without sudo/root:
$ docker version --format '{{.Server.Version}}'
1.9.1
Finally, a word of warning. Only trusted users should be allowed to control your Docker daemon. See https://docs.docker.com/engine/security/security/.
(But of course, the same is true for sudo - only trusted users should be in the wheel group.)
You need to add yourself to the docker group and activate the group (by logging out and in again or running newgrp docker) to run docker commands. The error message is simply misleading.
sudo usermod -aG docker [username]
then logout then log back in