[FEATURE] Add support for `exclusive_repos`

[synackd]

Is your feature request related to a problem? Please describe.
Yes. When specifying different base repos (e.g. custom BaseOS and Appstream Rocky repos), the stock repos (e.g. rocky.repo, rocky-addons.repo, rocky-devel.repo, and rocky-extras.repo) are still present in /etc/yum.repos.d/.

As an example, when trying to add Rocky 9.5 repos:

repos:
  - alias: 'Rocky_9.5_BaseOS'
    url: 'https://dl.rockylinux.org/vault/rocky/9.5/BaseOS/x86_64/os/'
    gpg: 'https://dl.rockylinux.org/pub/rocky/RPM-GPG-KEY-Rocky-9'
  - alias: 'Rocky_9.5_AppStream'
    url: 'https://dl.rockylinux.org/vault/rocky/9.5/AppStream/x86_64/os/'
    gpg: 'https://dl.rockylinux.org/pub/rocky/RPM-GPG-KEY-Rocky-9'

These repos get added alongside the existing stock repos and a package version not in the repos specified (kernel-5.14.0-570.21.1.el9_6.x86_64 for Rocky 9.6) is installed, which is not the desired behavior:

$ podman run --tls-verify=false --rm -it <image_fqcn> bash
[root@8cbdd613b58a ~]# ls /etc/yum.repos.d
dl.rockylinux.org_vault_rocky_9.5_AppStream_x86_64_os_.repo  dl.rockylinux.org_vault_rocky_9.5_BaseOS_x86_64_os_.repo  rocky-addons.repo  rocky-devel.repo  rocky-extras.repo  rocky.repo
[root@8cbdd613b58a ~]# rpm -q kernel
warning: Found bdb_ro Packages database while attempting sqlite backend: using bdb_ro backend.
kernel-5.14.0-570.21.1.el9_6.x86_64

The repos in /etc/yum.repos.d/rocky*.repo use $releasever, which pulls the latest Rocky 9 packages, which is undesired in this case.

Describe the solution you'd like
It would be useful to have an option, e.g. exclusive_repos, that clears all of the existing repos before setting the ones specified in the image-builder config. That way, exclusive_repos could be used for custom base repos in a root image config and repos could be used to add on to these in children image layers.

Describe alternatives you've considered
Having a remove_repos option to remove repos (similar to remove_packages), but this seems a bit more cumbersome than exclusive_repos.

Additional context
Testing using Rocky 9.5-based image configs.

[synackd]

I tried to solve this using:

remove_packages
- rocky-repos

since rocky-repos gets installed with the Minimal Install group; however, this directive happens after the package installation which defeats the purpose of removing the package in the first place (since it removes the repos the packages were installed from).

Having package/group installation/removal presents a sort of chicken-and-egg problem since, in the case of Rocky, the Minimal Install group is required to build the base filesystem, but it installs repos we need to remove (if wanting to use purely custom repos) to install other packages.

A workaround would be to separate the base Rocky layer into two layers:

1. The layer that installs the custom repos, installs Minimal Install, then removes rocky-repos
2. The layer that installs the actual packages that are needed

However, it would be easier to maintain one image layer instead of two for a single desired layer.

I'm not sure what the correct way to go about this is.

[travisbcotton]

I think we had the same thought.
I think if we don't use the default repo location then we might be able to skip repos installed by distros.

#24