Set PATH in Dockerfile "shell"
So that the built binary can be used as `docker-app`
instead of `bin/docker-app`:
```
$ make -f docker.Makefile shell
bash-4.4# make bin/docker-app
bash-4.4# docker-app version
Version:      v0.3.0-57-g77a969d-dirty
Git commit:
Built:        Wed Jul 18 13:06:12 2018
OS/Arch:      linux/amd64
Experimental: off
Renderers:    none
```
@@ -18,6 +18,7 @@ RUN curl -Ls https://download.docker.com/linux/static/$DOCKERCLI_CHANNEL/x86_64/
WORKDIR /go/src/github.com/docker/app/
FROM build AS dev
+ENV PATH=${PATH}:/go/src/github.com/docker/app/bin/
ARG DEP_VERSION=v0.4.1
RUN curl -o /usr/bin/dep -L https://github.com/golang/dep/releases/download/${DEP_VERSION}/dep-linux-amd64 && \
chmod +x /usr/bin/dep
