Improve Dockerfile for 4g example

Signed-off-by: Victor Morales <[email protected]>

examples/4g-network/sidecar-nse/Dockerfile

@@ -1,15 +1,17 @@
-FROM golang:alpine as build
-RUN apk --no-cache add git
+FROM golang:1.15.1-alpine3.12 as build
+
 ENV PACKAGEPATH=github.com/networkservicemesh/networkservicemesh/
 ENV GO111MODULE=on
 
-RUN mkdir /root/networkservicemesh
-ADD ["go.mod","/root/networkservicemesh"]
 WORKDIR /root/networkservicemesh/
+
+RUN apk --no-cache add git=2.26.2-r0
+COPY go.mod .
 RUN go mod download
 
-ADD [".","/root/networkservicemesh"]
-RUN CGO_ENABLED=0 GOOS=linux go build -ldflags '-extldflags "-static"' -o /go/bin/sidecar-nse ./examples/4g-network/sidecar-nse/cmd/main.go
-FROM alpine as runtime
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -ldflags '-extldflags "-static"' -o /go/bin/sidecar-nse ./examples/4g-network/sidecar-nse/cmd/...
+
+FROM alpine:3.12 as runtime
 COPY --from=build /go/bin/sidecar-nse /bin/sidecar-nse
 ENTRYPOINT ["/bin/sidecar-nse"]