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

[mboersma]

This fixes the problem when doing "make build" using Docker 1.3.2. Docker began erroring on "docker cp source ." as of this commit: moby/moby@31d1d73. Luckily the workaround is not difficult and is backward-compatible.

See also b5ba7c4 for a similar change we had to make in deis-builder.

fixes #2594

[gabrtv]

One day we'll have build-env / runtime-env isolation inside a single Dockerfile and this will all go away (see moby/moby#7149). Until then, LGTM.

[bacongobbler]

hmm, now the question arises: is 'pwd' or $PWD more portable? One would assume that both act the exact same way. Research time!

EDIT: http://askubuntu.com/a/476633 indicates that both will act the same way in either case (pwd just prints the value of $PWD), however $PWD probably reduces processing time by a few milliseconds since we don't fork the process.

This was more of a educational exercise than anything, not nitpicking your PR.

LGTM!

[mboersma]

@bacongobbler I thought you might bring that up. 💭 I can't tell if either is more portable; they both seem POSIXy and supported by bash and sh. So I am easily persuaded by any good argument here. If we use $PWD elsewhere in Deis, that's good enough for me.

[carmstrong]

I actually prefer pwd here - using $PWD makes me worry that it'll be evaluated before we expect it to be.