Check that your date on your machine is correct. I had this problem when one of my servers was showing a time of August 12, 2010! By fixing the time, the SSL stuff worked fine.
[x yum.repos.d]$ sudo yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo
Loaded plugins: fastestmirror
adding repo from: https:download.docker.com/linux/centos/docker-ce[dot]repo
grabbing file https:download.docker.com/linux/centos/docker-ce[dot]repo to /etc/yum.repos.d/docker-ce[dot]repo
Could not fetch/save url https:download.docker.com/linux/centos/docker-ce[dot]repo to file /etc/yum.repos.d/docker-ce.repo: [Errno 14] curl#35 - “Peer received a valid certificate, but access was denied.”