tests/docker: Install Ubuntu images noninteractively

We correctly use the DEBIAN_FRONTEND environment variable on
the Debian images, but forgot the Ubuntu ones are based on it.

Since building docker images is not interactive, we need to
inform the APT tools about it using the DEBIAN_FRONTEND
environment variable (we already use it on our Debian images).

This fixes:

  $ make docker-image-ubuntu V=1
  [...]
  Setting up tzdata (2019b-0ubuntu0.19.04) ...
  debconf: unable to initialize frontend: Dialog
  debconf: (TERM is not set, so the dialog frontend is not usable.)
  debconf: falling back to frontend: Readline
  Configuring tzdata
  ------------------

  Please select the geographic area in which you live. Subsequent configuration
  questions will narrow this down by presenting a list of cities, representing
  the time zones in which they are located.

    1. Africa      4. Australia  7. Atlantic  10. Pacific  13. Etc
    2. America     5. Arctic     8. Europe    11. SystemV
    3. Antarctica  6. Asia       9. Indian    12. US
  Geographic area: 12
  [HANG]

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190711124805.26476-1-philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>