support/docker: rename the Dockerfile and its accompanying file

Currently, we only generate one docker image, for use by the gitlab-ci
infra.

However, users have expressed an interest in using that image for other
uses, like:

  - reproducing build failures as reported by the gitlab-ci jobs;

  - having a ready-to-use base image to use as-is, or as a template for
    further customisations.

Additionally, some users have reported the need for similar base images,
but based off different distros, with the same goals as above, plus:

  - reproducing build failures reported by users on other distros.

As a first step toward providing such images, rename our Dockerfile to
include the name and version of the distro it is based off.

We add a symlink so that we still have a 'base' image that we can
reference from the gitlab-ci infra.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/docker/Dockerfile.base

@@ -0,0 +1 @@
+Dockerfile.debian-stretch

support/docker/Dockerfile.debian-stretch

@@ -1,6 +1,9 @@
+# vi: ft=dockerfile
 # This Dockerfile generates the docker image that gets used by Gitlab CI
 # To build it (YYYYMMDD.HHMM is the current date and time in UTC):
-#   sudo docker build -t buildroot/base:YYYYMMDD.HHMM support/docker
+#   sudo docker build -t buildroot/base:YYYYMMDD.HHMM \
+#                     -f support/docker/Dockerfile.debian-stretch \
+#                     support/docker
 #   sudo docker push buildroot/base:YYYYMMDD.HHMM
 
 # We use a specific tag for the base image *and* the corresponding date
@@ -16,7 +19,7 @@ description="Container with everything needed to run Buildroot"
 ENV DEBIAN_FRONTEND noninteractive
 
 # This repository can be a bit slow at times. Don't panic...
-COPY apt-sources.list /etc/apt/sources.list
+COPY debian-stretch.apt-sources.list /etc/apt/sources.list
 
 # The container has no package lists, so need to update first
 RUN dpkg --add-architecture i386 && \