https://medium.com/@ryanwhocodes/docker-copy-dockerfile-best-practices-503704bee69f

Docker has two similar Dockerfile instructions, COPY and ADD, which are both used to include files in an image. This article will explain why it is best practice to use COPY rather than ADD, unless you want to auto-extract a local tar file into an image.

This Dockerfile instruction copies one or many local files or folders into the destination within your Docker image.