support/docker: use a fixed Debian snapshot

Since we're now using a specific base image tag, we need to also use a
specific, stable repository to get additional packages from for this
image.

As such, use the Debian snapshot that matches the base image.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>