Sometimes you need to make changes to ADD-ed or COPY-ied files from Docker container processes. In these cases you may need to make something like:

COPY . /app-root
RUN chown -R myuser /app-root

This means you have to iterate twice over all files in /app-root, which may add a few seconds to every build for big hierarchies.

As of Docker 17.09, you can finally replace the chown command with a dedicated flag for COPY:

COPY --chown=myuser . /app-root

The same flag works for ADD. More information can be found in the PR. Unfortunately, chmod is not supported yet.