chore: upgrade golang to 1.16 (#103)

Dockerfile

@@ -1,6 +1,11 @@
+FROM golang:1.16 as builder
+WORKDIR /go/src/app
+COPY . .
+RUN make manager
+
 # Use distroless as minimal base image to package the manager binary
 # Refer to https://github.com/GoogleContainerTools/distroless for more details
 FROM gcr.io/distroless/static:latest
-COPY manager .
+COPY --from=builder /go/src/app/manager .
 USER 1000
 ENTRYPOINT ["/manager"]