support/docker: use a known base distribution

Currently, we are using debian:stable, which is subject to change with
time, as new stable versions of Debian are released/updated.

Use the latest tagged stable release, stretch-20171210 as of today, as
the base distribution to use.

This will ease reproducible builds in the future.

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>

support/docker/Dockerfile

@@ -3,7 +3,7 @@
 #   sudo docker build -t buildroot/base support/docker
 #   sudo docker push buildroot/base
 
-FROM debian:stable
+FROM debian:stretch-20171210
 LABEL maintainer="Buildroot mailing list <buildroot@buildroot.org>" \
       vendor="Buildroot" \
 description="Container with everything needed to run Buildroot"