Build grpc_health_probe and re-order dockerfile layers

Dockerfile

@@ -2,16 +2,15 @@
 FROM golang:alpine AS build-env
 ADD . /src
 WORKDIR /src
-ENV GOOS=linux
+RUN apk add git
+RUN go get github.com/grpc-ecosystem/grpc-health-probe@v0.3.0
+ENV GO111MODULE=on
 RUN go build -mod=vendor -ldflags '-extldflags "-static"' -o geras ./cmd/geras/main.go
 
 # final stage
 FROM alpine
 WORKDIR /bin
 COPY --from=build-env /src/geras /bin/
-ARG GRPC_HEALTH_PROBE_REPO_URL=https://github.com/grpc-ecosystem/grpc-health-probe
-RUN GRPC_HEALTH_PROBE_VERSION=v0.3.0 && \
-    wget -qO/bin/grpc_health_probe ${GRPC_HEALTH_PROBE_REPO_URL}/releases/download/${GRPC_HEALTH_PROBE_VERSION}/grpc_health_probe-linux-amd64 && \
-    chmod +x /bin/grpc_health_probe
+COPY --from=build-env /go/bin/grpc-health-probe /bin/
 USER 1000
 ENTRYPOINT ["./geras"]