try
systemctl start docker.service
              
              
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          In case anyone finds it handy in future:
On CentOS 7 try linking the non-existent docker.io binary to the existing docker binary as follows:
ln -s /usr/bin/docker /usr/bin/docker.io
This did it for me just now…
              
              
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          Worked. Somehow this should come up as the right answer.
              
              
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          Exactly. If it says docker isn’t running, just do a service docker start and move on. That’s what I did.