x509: certificate signed by unknown authority – both with docker and with github

As mentioned in crypto/x509/root_unix.go, Go (which is what Docker uses) will check CA certificates in

"/etc/ssl/certs/ca-certificates.crt",     // Debian/Ubuntu/Gentoo etc.
"/etc/pki/tls/certs/ca-bundle.crt",       // Fedora/RHEL
"/etc/ssl/ca-bundle.pem",                 // OpenSUSE
"/etc/ssl/cert.pem",                      // OpenBSD
"/usr/local/share/certs/ca-root-nss.crt", // FreeBSD/DragonFly
"/etc/pki/tls/cacert.pem",                // OpenELEC
"/etc/certs/ca-certificates.crt",         // Solaris 11.2+

Make sure those files are available and not corrupted.

There can be also sporadic issue with the CDN, as in this comment:

because now it works :+1: . It must be a amazon edge isssue

The last thread also includes the following check:

The user reporting the issue either has non of those files or those files don’t include the rapidssl cert.
We could ask them to send us those files and check if the certificate is included.
The user may also try this:

openssl s_client -showcerts -verify 32 -connect index.docker.io:443

If that fails, the certificates are missing.

Regarding GitHub, be aware it is under a massive DDoS attack at the moment, which could have other side-effects beside the certificate issue.