frontend: optimize dockerfile

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

src/frontend/Dockerfile

@@ -1,23 +1,29 @@
 FROM golang:1.10-alpine as builder
 RUN apk add --no-cache ca-certificates git
 WORKDIR /go/src/frontend
-COPY . .
 
-# download known dependencies
+# fetch known dependencies for caching
 RUN go get -d github.com/google/uuid \
     github.com/gorilla/mux \
     google.golang.org/grpc \
     google.golang.org/grpc/codes \
     google.golang.org/grpc/status
 
-# other dependencies might not have listed above
+# copy go-only part of the build
+COPY *.go ./
+COPY ./genproto ./genproto
+COPY ./money    ./money
+
+# fetch other dependencies might not have listed above (ideally noop)
 RUN go get -d ./...
-RUN go build -o /frontend .
+RUN go install .
+
+# ---
 
 FROM alpine as release
 RUN apk add --no-cache ca-certificates
 WORKDIR /frontend
-COPY --from=builder /frontend /frontend/server
+COPY --from=builder /go/bin/frontend /frontend/server
 COPY ./templates ./templates
 COPY ./static ./static
 EXPOSE 8080