*/Dockerfile.template: Replace ADD with COPY

The COPY command landed in Docker v0.12.0 as a less-magical version of
ADD [1].  The Docker folks have been replacing assorted ADD occurences
with COPY since then [2,3,4].  The only caveat is that it won't work
on older versions of Docker, but I agree with a bunch of folks that
that's something best dealt with by upgrading your Docker version [5].

[1]: https://github.com/docker/docker/pull/6089
[2]: https://github.com/docker/docker/issues/6912
[3]: https://github.com/docker/docker/pull/6941
[4]: https://github.com/docker/docker/pull/6942
[5]: https://github.com/docker/docker/pull/7471