The releasever variable needs to be set for certain image builds when creating images for EUS and E4S. If we expose it as a customization, it would save us from needing to create image variants internally.

This would be in line with previous work that moved more configurations into Pungi (HMS-7026), which lets us define fewer internal image types and control the creation of variants externally.

I'm not entirely sure about the form of the customization:

• General dnf vars (key=value)?
• A dnf vars customization with predefined keys? We would initially only support releasever and keep open the possibility of adding more when we need them.
• Specific option for "pinning" releasever (boolean that simply sets releasever=<distro version>)?