permanent cache mounts

[nrdxp]

Use case
It has been very difficult to predict when a CACHE mount in our project gets invalidated or wiped. Since it is a Rust based project and we use it primarily to cache the target dir and we also use cargo sweep to clean up old artifacts over a certain age we are okay with it basically never being invalidated or cleared for any reason (by earthly at least).

It would be nice if there were some sort of CACHE --permanent or something along those lines to ensure that this particular mount is never wiped or invalidated.

[alexcb]

This may be difficult to implement since it will require several changes to our buildkit fork; if anyone else would like this feature please upvote it with a 👍