Add documentation of --chmod flag in ADD and COPY instructions to builder.md

[rcjsuen]

builder.md does not mention the new --chmod flag that was added to ADD and COPY instructions in Docker CE 20.10.

[thernstig]

Isn't this a big oversight that this has not been documented yet?

[sam-thibault]

This file has moved to the BuildKit repository at https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/reference.md

[sam-thibault]

I created an issue on buildkit since the file is there now:

• Add documentation of --chmod flag in ADD and COPY instructions to builder.md moby/buildkit#3721