docker: Use a stable snapshot for Debian Sid

docker: Use a stable snapshot for Debian Sid

The Debian Sid repository is not garanteed to be stable, as his
'unstable' name suggest :)
To allow quick testing, packages are pushed various time a day,
which my be annoying when trying to use it for stable development
(which is not recommended, but Sid provides edge packages we use
for testing).

Debian provides repositories snapshots which are suitable for our
use. Pick a recent date that works. When required, update to newer
releases will be easy.

This fixes current issues with this image:

  $ make docker-image-debian-sid
  [...]
  The following packages have unmet dependencies:
   build-essential : Depends: dpkg-dev (>= 1.17.11) but it is not going to be installed
   git : Depends: perl but it is not going to be installed
         Depends: liberror-perl but it is not going to be installed
   pkg-config : Depends: libdpkg-perl but it is not going to be installed
   texinfo : Depends: perl (>= 5.26.2-6) but it is not going to be installed
             Depends: libtext-unidecode-perl but it is not going to be installed
             Depends: libxml-libxml-perl but it is not going to be installed
  E: Unable to correct problems, you have held broken packages.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[AJB: also tweak FROM to a earlier snapshot]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>