How to completely uninstall docker

To completely uninstall Docker:

Step 1

dpkg -l | grep -i docker

To identify what installed package you have:

Step 2

sudo apt-get purge -y docker-engine docker docker.io docker-ce docker-ce-cli
sudo apt-get autoremove -y --purge docker-engine docker docker.io docker-ce  

The above commands will not remove images, containers, volumes, or user created configuration files on your host. If you wish to delete all images, containers, and volumes run the following commands:

sudo rm -rf /var/lib/docker /etc/docker
sudo rm /etc/apparmor.d/docker
sudo groupdel docker
sudo rm -rf /var/run/docker.sock

You have removed Docker from the system completely.

If you are on Ubuntu, I find it much easier to uninstall docker if it is installed with snap. You simply do:

sudo snap remove docker

or, to avoid it creating back-up data for a snap you no longer require:

sudo snap remove --purge docker

and to delete all related files,

sudo find / -name "*docker*" -exec `rm -rf` {} +

IMPORTANT UPDATES

As mentioned in the comments,

The second part is not needed and just dangerous, at least run it without -exec `rm -rf` {} + first

and

The second part could delete files that belong to different packages, e.g. /usr/share/vim/vim80/syntax/dockerfile.vim

So better to see what is going to be deleted first.

Add docker-ce-cli package also while doing purge of docker

sudo apt-get purge -y docker-engine docker docker.io docker-ce docker-ce-cli