Fix Dockerfile to add missing apt-update before apt-install

There are some issues in stage2 when installing apt packages due
to the use of an old apt cache. This commit fixes it by doing apt
update before apt install.

Change-Id: I539155280a2caae65442c7f0805f3232b505612d
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>