Local exporter: Export individual files

[jleben]

Is there a way to export only parts of the output filesystem to local filesystem?

I haven't found a buildctl option to do that.

I guess I could theoretically define a graph that ends with a scratch state and mounts parts of another state. However, the client library doesn't seem to provide AddMount for State, but only for ExecState, and I don't see a trivial way to form the latter from the former. In any case, such an additional mounting step would seem like an avoidable inconvenience to me...