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>

support/docker/Dockerfile

@@ -3,7 +3,11 @@
 #   sudo docker build -t buildroot/base support/docker
 #   sudo docker push buildroot/base
 
+# We use a specific tag for the base image *and* the corresponding date
+# for the repository., so do not forget to update the apt-sources.list
+# file that is shipped next to this Dockerfile.
 FROM debian:stretch-20171210
+
 LABEL maintainer="Buildroot mailing list <buildroot@buildroot.org>" \
       vendor="Buildroot" \
 description="Container with everything needed to run Buildroot"
@@ -11,6 +15,9 @@ description="Container with everything needed to run Buildroot"
 # Setup environment
 ENV DEBIAN_FRONTEND noninteractive
 
+# This repository can be a bit slow at times. Don't panic...
+COPY apt-sources.list /etc/apt/sources.list
+
 # The container has no package lists, so need to update first
 RUN dpkg --add-architecture i386 && \
     apt-get update -y && \

support/docker/apt-sources.list

@@ -0,0 +1,4 @@
+# Latest just before 20171210T000000Z:
+deb [check-valid-until=no] http://snapshot.debian.org/archive/debian/20171209T220346Z/ stretch main
+deb [check-valid-until=no] http://snapshot.debian.org/archive/debian/20171209T220346Z/ stretch-updates main
+deb [check-valid-until=no] http://snapshot.debian.org/archive/debian-security/20171209T224618Z/ stretch/updates main