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

This fixes the problem when doing "make build" using Docker 1.3.2. Docker began erroring on "docker cp source ." as of this commit: https://github.com/docker/docker/commit/31d1d733037b22591e2dd2edfe6c4d2d4b8086cc
Luckily the workaround is not difficult.

See also b5ba7c498ff7dc18b01cdb4746caa080672c10f4 for a similar change we had to make in **deis-builder**.