Description
I've bumped into #34263 being merged and it hit me that --chmod would be nice in conjunction with this feature.

I frequently build Linux images from a Windows machine and I'm kind of annoyed with the 755 mode the added/copied files end up with.

ADD/COPY with --chown and --chmod together will be a readable and maintainable notation delivering precise control over permissions.

I'm aware of moby/buildkit#4242, but I don't think this issue is a duplicate (that one seems to be stale).