The Docker daemon binds to a Unix socket instead of a TCP port.
By default that Unix socket is owned by the user root and other users can only access it using sudo. The Docker daemon always runs as the root user.

If you don’t want to preface the docker command with sudo, create a Unix group called docker and add users to it. When the Docker daemon starts, it creates a Unix socket accessible by members of the docker group.

To create the docker group and add your user:

1. Create the docker group

    sudo groupadd docker
   

2. Add your user to the docker group

    sudo usermod -aG docker $USER
   

3. Log out and log back in so that your group membership is re-evaluated.

   If testing on a virtual machine, it may be necessary to restart the virtual machine for changes to take effect.

   On a desktop Linux environment such as X Windows, log out of your session completely and then log back in.

   On Linux, you can also run the following command to activate the changes to groups:

    newgrp docker 
   

4. Verify that you can run docker commands without sudo. The below command downloads a test image and runs it in a container. When the container runs, it prints an informational message and exits

   docker run hello-world
   

If you initially ran Docker CLI commands using sudo before adding your user to the docker group, you may see the following error, which indicates that your ~/.docker/ directory was created with incorrect permissions due to the sudo commands.

   WARNING: Error loading config file: /home/user/.docker/config.json -
   stat /home/user/.docker/config.json: permission denied

To fix this problem, either remove the ~/.docker/ directory (it is recreated automatically, but any custom settings are lost), or change its ownership and permissions using the following commands:

sudo chown "$USER":"$USER" /home/"$USER"/.docker -R

sudo chmod g+rwx "$HOME/.docker" -R

All other post installation steps for docker on linux can be found here https://docs.docker.com/engine/install/linux-postinstall/