Build a docker container

Use debian/distroless (glibc) as a base which in general gives
slightly higher performance than alpine (musl) while trading
off container size (about 10mb).

The build phase makes the assumption that any capnproto-files
are pre-processed before building in-docker.

Dockerfile

@@ -0,0 +1,12 @@
+FROM golang:1.12 as builder
+WORKDIR /go/src/github.com/cloudflare/cloudflared/
+RUN apt-get update && apt-get install -y --no-install-recommends upx
+# Run after `apt-get update` to improve rebuild scenarios
+COPY . .
+RUN make cloudflared
+RUN upx --no-progress cloudflared
+
+FROM gcr.io/distroless/base
+COPY --from=builder /go/src/github.com/cloudflare/cloudflared/cloudflared /usr/local/bin/
+ENTRYPOINT ["cloudflared", "--no-autoupdate"]
+CMD ["version"]