With Docker version 17.09.0-ce, build afdb6d4, it was working for me, but after yum update to get Docker version 18.06.0-ce, build 0ffa825, I had the same problem.
Could you please check your /etc/systemd/system/docker.service.d/https-proxy.conf ? I think you have configured like this :
HTTPS_PROXY=https://user:pwd@proxy:port
But now, try to configure HTTPS_PROXY with http protocol instead of https protocol :