SSL certificate problem

got error when clone something in docker

fatal: unable to access 'https://github.com/rails/rails/': SSL certificate problem: self signed certificate in certificate chain

works after run

openssl s_client -showcerts -servername github.com -connect github.com:443 </dev/null 2>/dev/null | sed -n -e '/BEGIN\ CERTIFICATE/,/END\ CERTIFICATE/ p'  > github-com.pem