dockerfile: add python link to python3

Change-Id: I19b9a8ddfff77e37c0e94213be0fcae3ae3a951e

docker/Dockerfile

@@ -25,6 +25,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 	m4 \
 	pkg-config \
 	software-properties-common \
+	python-is-python3 \
 	ssh \
 	unzip \
 	wget \