make COPY --chown work without modifyfs

[hellmanj]

Is your feature request related to a problem? Please describe.
Similar to #185 , we need to use COPY --chown to change file ownership while creating our image. In our organization, we are not permitted to run makisu in a container or use chroot (main reasons we are drawn to makisu), so using --modifyfs isn't really an option for us.

Describe the solution you'd like
Please make COPY --chown work without --modifyfs. I heard from @yiranwang52 there is already a TODO in the code to make this work...

Describe alternatives you've considered
Would love to hear some :)

Additional context
Sample Dockerfile which reproduces the issue:

FROM alpine
COPY --chown=0:0 . /srv

Thanks! (and thanks for making Makisu!)