I've been having this problem on Fedora 23 with docker 1.9.1 build ee06d03/1.9.1. When I would use docker pull, it would give me a cert error:
Not sure why docker can't just use the system cert bundle. Looking at the code: https://github.com/docker/docker/blob/1061c56a5fc126a76344ea9dca9aa5f5e75eb902/registry/registry.go#L102 docker looks for /etc/docker/certs.d/$hostname and looks for a CA cert bundle in that directory. So I just did this:
# docker pull some/image:tag Trying to pull repository docker.io/some/image ... failed Error while pulling image: Get https://index.docker.io/v1/repositories/some/image/images: x509: certificate signed by unknown authority
Now docker pull works fine for the Dockerhub repo.
# cd /etc/docker/certs.d mkdir docker.io cd docker.io ln -s /etc/pki/tls/certs/ca-bundle.crt ln -s /etc/pki/tls/certs/ca-bundle.trust.crt systemctl restart docker