fix(router/Makefile): use pwd since "docker cp" no longer allows "."

This fixes the problem when doing "make build" using [Docker 1.3.2](https://docs.docker.com/release-notes/#version-132). Docker began erroring on "docker cp source ." as of this commit: https://github.com/docker/docker/commit/31d1d733037b22591e2dd2edfe6c4d2d4b8086cc. Luckily the workaround is not difficult and is backward-compatible.

See also b5ba7c498ff7dc18b01cdb4746caa080672c10f4 for a similar change we had to make in **deis-builder**.

fixes #2594