How to repair docker or reinstall it?

Try this on ubuntu with docker-ce

sudo apt-get install --reinstall docker-ce

That was seen only with older versions of docker (issue 22343), but also more recently with 16.04.1 LTS, Docker version 1.12.1, build

It depends on which docker you are using, and on which host OS.

If you are on Ubuntu for instance, try removing docker completely (if you don't have any local work that you would want to keep)

sudo apt-get purge docker-engine
sudo apt-get autoremove --purge docker-engine
rm -rf /var/lib/docker # This deletes all images, containers, and volumes

Then install the latest docker 13.1 and see if the issue persists.

Again, this is only if you are ready to start again from a clean state, reinstalling Docker entirely.
Do not execute rm -rf /var/lib/docker without considering the consequences, or in a production environment(!).

rakeshz's answer using an apt-get install --reinstall is a safer first step.