Provide yum specific image building glue.

RHEL/Fedora/CentOS and other yum based distributions need specific yum customizations.

Customizations include caching of downloaded yum packages outside of the build chroot so that they can be reused by subsequent image builds. The cache increases image building speed when building multiple images, especially on slow connections. This is more effective than using an HTTP proxy as a yum cache since the same rpm from different mirrors is often requested.

Custom yum repository configurations can also be applied by defining DIB_YUM_REPO_CONF to a space separated list of repo configuration files. The files will be copied to /etc/yum.repos.d/ during the image build, and then removed at the end of the build. Each repo file should be named differently to avoid a filename collision.

Environment Variables for Module Selection during Image Creation

The following environment variable is used to select module streams to be enabled during an image build on Yum/DNF based distributions. Any existing stream for the given module is first disabled prior to enabling the specified stream.

#### DIB_DNF_MODULE_STREAMS This is a space-separated list of module streams to enable prior to any RPMs being installed.

Image Build Module Selection Example

When using Train release on RHEL/CentOS/Fedora, one must select the appropriate virt and container-tools module streams:

DIB_DNF_MODULE_STREAMS=’virt:8.2 container-tools:3.0’