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.
The yum repository can also be configured by defining DIB_YUM_REPO_PACKAGE as a yum available package or a URL to an rpm file. This package can install repo files with any associated keys and certificates.
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.
The environment variable is used to override default value in pre-install stage to install epel repository from custom source