(maint) Dockerfiles use `COPY` instead of `ADD` (#357)
(maint) Dockerfiles use `COPY` instead of `ADD` (#357)

- Use `COPY` because it doesn't take advantage of the `ADD` auto `.tar` extraction feature.

Taken from Docker's [Best practices](https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#add-or-copy) documentation.

Signed-off-by: Vladislav Doster <1c5f47fa25b6738e2708af6b8e940862eecd3438@gmail.com>
singularityhub/sregistry,singularityhub/sregistry,singularityhub/sregistry,singularityhub/sregistry