Use linked copies in Dockerfile

[lucacome]

From the docs https://github.com/moby/buildkit/blob/dockerfile/1.4.0/frontend/dockerfile/docs/syntax.md#linked-copies-copy---link-add---link

Enabling this flag in COPY or ADD commands allows you to copy files with enhanced semantics where your files remain independent on their own layer and don't get invalidated when commands on previous layers are changed.