Update building stuff

- Add a Makefile with common targets (binary, test, validate, ..)
- Use docker to build it (isolated)

And update circleci to work with the new Makefile..

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

Dockerfile

@@ -1,3 +1,3 @@
 FROM scratch
-ADD traefik /
-CMD ["/traefik"]
+COPY dist/traefik_linux-386 /traefik
+ENTRYPOINT ["/traefik"]