Many problems that appear to be Docker-related are often in fact caused by VirtualToolbox.

Before proceeding on the assumption that the problem lies with Docker, please see:

Error: "Couldn't connect to Docker daemon"

Problem

Docker commands report that they cannot connect to the Docker daemon: Cannot connect to the Docker daemon. Is the docker daemon running on this host? 

However, you're sure it is actually running.

Probable cause

The problem may be an issue with users and groups.

Solution

You need to add yourself to the docker group. 

What to do

See this StackExchange discussion for more.

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