Use numeric uid:gid in Dockerfile to support Tanzu's PSP (#720)

- Fixes issue
  "Error: container has runAsNonRoot and image has non-numeric user (nonroot), cannot verify user is non-root"

Author: lhotari

Dockerfile

@@ -24,6 +24,6 @@ RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 GO111MODULE=on go build -a -o manager
 FROM gcr.io/distroless/static:nonroot
 WORKDIR /
 COPY --from=builder /workspace/manager .
-USER nonroot:nonroot
+USER 65532:65532
 
 ENTRYPOINT ["/manager"]