Build docproxy with Docker

Full details on this change: cloud-gov/product#3216 (comment)

docproxy/Dockerfile

@@ -0,0 +1,14 @@
+ARG builder_image
+ARG base_image
+
+# Builder: Build the Go app using an image with the Go toolchain installed.
+FROM ${builder_image} AS build
+ADD . /app
+WORKDIR /app
+RUN go build .
+
+# Create the final image based on our hardened base image.
+FROM ${base_image}
+COPY --from=build /app/docproxy /app/
+
+CMD [ "docproxy" ]