support/docker: update the stretch image

The current stretch image we are using on the maintenance branches still
has the old Let's Encrypt certificate that expired a few weeks ago,
causign a lot of download errors:

Suggested-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/docker/Dockerfile

@@ -4,9 +4,9 @@
 #   sudo docker push buildroot/base:YYYYMMDD.HHMM
 
 # 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
+# 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
+FROM debian:stretch-20211115
 
 LABEL maintainer="Buildroot mailing list <buildroot@buildroot.org>" \
       vendor="Buildroot" \

support/docker/apt-sources.list

@@ -1,4 +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
+# Latest just before 20211115T000000Z:
+deb [check-valid-until=no] http://snapshot.debian.org/archive/debian/20211114T204852Z/ stretch main
+deb [check-valid-until=no] http://snapshot.debian.org/archive/debian/20211115T000000Z/ stretch-updates main
+deb [check-valid-until=no] http://snapshot.debian.org/archive/debian-security/20211114T121535Z/ stretch/updates main