===
yum
===
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 RHEL8.  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 on RHEL8.2, one must select the appropriate virt and
container-tools module streams:

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