Docker: Are you trying to connect to a TLS-enabled daemon without TLS?

You can check if it has started by running the command ps -ef. You can also grep it to docker if you want to reduce the number of results(using | grep docker). If its of not running, execute

sudo service docker start

OR if it still doesn't work then

You can refer this link

docker docs

You can run docker run -d or docker run -d & so that you can use the same terminal or even close it if. It will set the value to true, so your container will run in "detached" mode, in the background.

You can also auto start it when your OS starts using update-rc.d servicename defaults or you can also refer to the links below, where you have to give docker as service name and your defaults.

Some more links to refer - auto start,upstart

These are different ways of doing it.

I deployed docker on CentOS 7, and my friend first helped me check whether the process/service docker is actually running or not. He used the command ps -ef | grep docker. It turned out that the process wasn't running at all. Finally, he started this process by using the command service docker start. In the end, the problem was solved.

Try installing apparmor

sudo apt install apparmor

There are other things to look for in this answer too.