A case to use COPY rather than ADD

[junaruga]

This related to multiarch/qemu-user-static#66 .

seeing

https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#add-or-copy
Although ADD and COPY are functionally similar, generally speaking, COPY is preferred.
...
For other items (files, directories) that do not require ADD’s tar auto-extraction capability, you should always use COPY.

Maybe this code is the case to change from ADD to COPY.
https://github.com/multiarch/centos/blob/master/update.sh#L103

ADD qemu-${qemu_arch}-static /usr/bin