Dockerfile: install iproute2 in the container image.
Fixes
@@ -34,5 +34,5 @@ COPY . .
RUN go install -v ./cmd/...
FROM alpine:3.11
-RUN apk add --no-cache ca-certificates iptables
+RUN apk add --no-cache ca-certificates iptables iproute2
COPY --from=build-env /go/bin/* /usr/local/bin/
