add minor Dockerfile improvements

Signed-off-by: Tariq Ibrahim <[email protected]>

Author: tariq1890

Dockerfile

@@ -15,9 +15,9 @@ WORKDIR /go/src/github.com/prometheus/client_golang/examples/simple
 RUN CGO_ENABLED=0 GOOS=linux go build -a -tags netgo -ldflags '-w'
 
 # Final image.
-FROM prom/busybox
+FROM quay.io/prometheus/busybox:latest
 LABEL maintainer="The Prometheus Authors <[email protected]>"
 COPY --from=builder /go/src/github.com/prometheus/client_golang/examples/random \
     /go/src/github.com/prometheus/client_golang/examples/simple ./
 EXPOSE 8080
-CMD echo Please run an example. Either /random or /simple
+CMD ["echo", "Please run an example. Either /random or /simple"]