Using COPY or ADD command to add a file and then changing ownership for the files introduces an extra layer and can take a long time when changing ownership for a lot of files.

FROM xxxx

COPY src/ /opt/src/
RUN chown -R www-data:www-data /opt/src/

Instead Docker ADD and COPY commands support --chown arguments which can be used to add files to the image and assign correct ownerships in a single step and this reduces an extra layer and produces a smaller image.

FROM xxxx

COPY --chown=www-data:www-data src/ /opt/src/

Note: This feature is only available in 17.09 or later versions and is only supported in Linux container images

References:

1. Docker docs