Update always fail on ubuntu 14.04

I want to install docker on ubuntu 14.04 according to the doc Get Docker for Ubuntu. Link: https://docs.docker.com/engine/installation/linux/ubuntu/

All steps are OK until sudo apt-get update. It always fail.

Ubuntu use proxy, I think maybe the problem is about this. But I do not know how to solve it.

W: Failed to fetch https://download.docker.com/linux/ubuntu/dists/trusty/stable/binary-amd64/Packages  server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none