docker: ubuntu: Update the package list before installing new ones

Since docker caches the different layers, updating the package
list does not invalidate the previous "apt-get update" layer,
and it is likely "apt-get install" hits an outdated repository.

See https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#apt-get

This fixes:

  $ make docker-image-ubuntu V=1
  ./tests/docker/docker.py build qemu:ubuntu tests/docker/dockerfiles/ubuntu.docker   --add-current-user
  Sending build context to Docker daemon  3.072kB
  [...]
  E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/m/mesa/libgles2-mesa_17.0.7-0ubuntu0.16.04.2_amd64.deb  404  Not Found
  E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/m/mesa/libgles2-mesa-dev_17.0.7-0ubuntu0.16.04.2_amd64.deb  404  Not Found
  E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
  The command '/bin/sh -c apt-get -y install $PACKAGES' returned a non-zero code: 100
  tests/docker/Makefile.include:40: recipe for target 'docker-image-ubuntu' failed
  make: *** [docker-image-ubuntu] Error 1

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/docker/dockerfiles/ubuntu.docker

@@ -1,7 +1,6 @@
 FROM ubuntu:16.04
 RUN echo "deb http://archive.ubuntu.com/ubuntu/ trusty universe multiverse" >> \
     /etc/apt/sources.list
-RUN apt-get update
 ENV PACKAGES flex bison \
     libusb-1.0-0-dev libiscsi-dev librados-dev libncurses5-dev libncursesw5-dev \
     libseccomp-dev libgnutls-dev libssh2-1-dev  libspice-server-dev \
@@ -13,6 +12,7 @@ ENV PACKAGES flex bison \
     libjemalloc-dev libcacard-dev libusbredirhost-dev libnfs-dev libcap-dev libattr1-dev \
     texinfo \
     gettext git make ccache python-yaml gcc clang sparse
-RUN apt-get -y install $PACKAGES
+RUN apt-get update && \
+    apt-get -y install $PACKAGES
 RUN dpkg -l $PACKAGES | sort > /packages.txt
 ENV FEATURES clang pyyaml