Docker on Ubuntu - not working

Today I've installed Docker into my Ubuntu Linux 15.04 according to official documentation (note here: after installation I've executed  sudo usermod -aG docker <my_username_here> because I want to execute Docker as a non-root user).

However, when I tried to do docker docker run hello-world I got:

ernestas@ernestas-VirtualBox:~$ docker run hello-world

A start job is running for dev-disk - Ubuntu Linux 15.04

Today I had a very akward problem with my Ubuntu Linux 15.04. During boot I got a message saying:

A start job is running for dev-disk

And a timer. As a result, the boot of Linux was delayed for about 1 min 30 s. So, what did I do about it?

I opened sudo vim /etc/fstab and found that the SWAP partition was described as: