Specifying debian mirror for your docker image

For people who use Debian as docker base image...

• Do not use ftp.debian.org as your apt line in docker image, use deb.debian.org instead. It chooses near mirror where users are
• If you use old Jessie, use cdn-fastly.deb.debian.org instead of deb.debian.org. Since old apt in Jessie cannot handle redirect
• Do not use httpredir.debian.org, it is obsolete