There are new features:

• All chroot (but rawhide) configs now contains best=1. This way DNF will always try to install latest package. If its dependence cannot be satisfied DNF will report an error. Without this DNF may silently install some older version which does not have broken deps. This is fine for regular user, but not for buildsystems, where maintainers usually want to use latest version. Note that this change may result in sudden build failure, which previously silently succedded. In this case, please check your BuildRequires and ask maintainers of those build deps to resolve broken dependency. This option was not added to rawhide chroots as there are broken dependencied very often. Additionaly option best=1 is used for repos passed to mockchain using -a option.
• new config for epel-7-aarch64 chroot
• You can use new variable hostname: config_opts['hostname'] = 'my.own.hostname' This unconditionally calls sethostname(), however variable use_container_host_hostname or %_buildhost macro can override this (on F25+).
• Use DNF on RHEL, when it is installed and configured RHBZ#1405783
• Temporary directories now use tmp.mock. prefix.
• Temporary directories are now removed even when buildroot is not cleaned.
• Add bash completion for .cfg files outside /etc/mock #20

There are some bugfixes:

• Handle working directories which contains spaces RHBZ#1389663
• Error: is not iterable RHBZ#1387895
• Delay mounting of user-defined mountpoints RHBZ#1386544
• Added example how to use more_buildreqs when you need more packages
• Added example how to use --plugin-option