tests/docker: update Debian Sid image

tests/docker: update Debian Sid image

While we are not currently using it we might as well keep the image
for later usage. So:

  - update to a more recent snapshot
  - clean up verbiage in commentary
  - remove duplicate shell from a merge failure

Suggested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
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>