I confirm this feature has not been previously requested

• I have searched the issues and this feature has not previously been requested

Describe the solution you'd like
An additional Source variant should be added, which includes instructions quickget can use in order to build an image. Image builds must be completely reproducible, so containers must be used, adding podman or docker as an optional dependency of quickemu. All releases to be built in this manner should be suffixed with -build, allowing quickget to easily skip them if the necessary tools aren't installed or they're disabled in another way (e.g. a compile time flag for quickget-rs).

GitHub packages can be used to distribute docker images, each of which should accept environment variables to select the image to be built. Unique docker images should only be created when a different source image is required for the build process. Scripts contained within each image should rely on distro-provided tools such as https://github.com/canonical/ubuntu-image or https://github.com/pop-os/iso if possible. Images must include json-formatted metadata that quickget_ci will read in order to produce configs based on them.