tests/docker: squash initial update and install step for debian9

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/docker/dockerfiles/debian9.docker

@@ -13,8 +13,8 @@ FROM debian:stretch-slim
 RUN cat /etc/apt/sources.list | sed "s/^deb\ /deb-src /" >> /etc/apt/sources.list
 
 # Install common build utilities
-RUN apt update
-RUN DEBIAN_FRONTEND=noninteractive apt install -yy eatmydata
+RUN apt-get update && \
+    DEBIAN_FRONTEND=noninteractive apt install -yy eatmydata
 RUN DEBIAN_FRONTEND=noninteractive eatmydata \
     apt install -y --no-install-recommends \
         bison \