This image serves as the official Fedora image for Fedora 22, Fedora 21 and as a semi-official image for Fedora 20 (heisenbug) and rawhide.
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The fedora:latest tag will always point to the latest stable release, currently Fedora 22. fedora:latest is now the same as fedora:22.
Fedora rawhide is available via fedora:rawhide and Fedora 20 via fedora:20 and fedora:heisenbug. Fedora 20 has now reached EOL and will receive no updates after the fix for CVE-2015-4000 (Logjam).
The metalink http://mirrors.fedoraproject.org is used to automatically select a mirror site (both for building the image as well as for the yum repos in the container image).
$ docker run fedora cat /etc/yum.repos.d/fedora.repo | grep metalink
metalink=https://mirrors.fedoraproject.org/metalink?repo=fedora-$releasever&arch=$basearch
metalink=https://mirrors.fedoraproject.org/metalink?repo=fedora-debug-$releasever&arch=$basearch
metalink=https://mirrors.fedoraproject.org/metalink?repo=fedora-source-$releasever&arch=$basearch