Wow, the COPY directive got a lot more powerful. I've been waiting for the --parent flag for years, while the --exclude argument is also a nice touch. Didn't know of the /./ pivot point before, but that's handy.
Before this, I've just been using a intermediary leaf stage within a multi-stage build process to copy the build context and filter the dependency lock files of the entire super project into a matching parent structure that I could then deterministically copy from.