[feature request] add options to specify where layers are stored localy

[mcarbonneaux]

in issue "#4638" we see echange about the imposibility to specify local localtion of the layers with "--cache-to" and "--cache-from"...

in ci/cd in general you can cache (gitlab cache) in the pipe line between jobs execution, but you need to kown where the data are stored and many build tool had option to specify where cache file are stored.

can you add the posibility to specify the local location with --cache-to and --cache-from ?

and at minima where the default location of the layers ?