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and it is likely "apt-get install" hits an outdated repository.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>