Request for optional SSHKEY make variable

[danmcp]

Re: https://github.com/stefwalter/drama-llama/blob/main/1-herd/Containerfile#L3

cc @kwozyman

[rhatdan]

Should we make SSHKEY a snippet and allow users to request it. Seems like it should be treated as an alternative to cloud-init.
Perhaps we could add something like

INCLUDES=sshkey,cloud-init and other Containerfile snippets.

https://gitlab.com/bootc-org/examples.git

[rhatdan]

@cgwalters WDYT?