Update Docker integration guide to prefer `COPY` over `ADD` for simple cases (#16883)
## Summary
Docker best practices recommend to use `COPY` when the additional
functionality of `ADD` is not used.
See:
- https://docs.docker.com/build/building/best-practices/#add-or-copy
-
https://www.docker.com/blog/docker-best-practices-understanding-the-differences-between-add-and-copy-instructions-in-dockerfiles/
## Test Plan
Docs only change