docker: Bump OS version to debian buster

Since we're planning on standardizing on Python 3.6+ [1] I think it is a good
idea to bump our own docker image to a Debian version that fulfills that
requirement as well.

[1] https://github.com/lightningd/plugins/issues/146

Dockerfile

@@ -5,7 +5,7 @@
 # * final: Copy the binaries required at runtime
 # The resulting image uploaded to dockerhub will only contain what is needed for runtime.
 # From the root of the repository, run "docker build -t yourimage:yourtag ."
-FROM debian:stretch-slim as downloader
+FROM debian:buster-slim as downloader
 
 RUN set -ex \
 	&& apt-get update \
@@ -45,7 +45,7 @@ RUN mkdir /opt/litecoin && cd /opt/litecoin \
     && tar -xzvf litecoin.tar.gz $BD/litecoin-cli --strip-components=1 --exclude=*-qt \
     && rm litecoin.tar.gz
 
-FROM debian:stretch-slim as builder
+FROM debian:buster-slim as builder
 
 ENV LIGHTNINGD_VERSION=master
 RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates autoconf automake build-essential git libtool python3 python3-mako wget gnupg dirmngr git gettext
@@ -80,7 +80,7 @@ RUN git clone --recursive /tmp/lightning . && \
 ARG DEVELOPER=0
 RUN ./configure --prefix=/tmp/lightning_install --enable-static && make -j3 DEVELOPER=${DEVELOPER} && make install
 
-FROM debian:stretch-slim as final
+FROM debian:buster-slim as final
 COPY --from=downloader /opt/tini /usr/bin/tini
 RUN apt-get update && apt-get install -y --no-install-recommends socat inotify-tools python3 python3-pip \
     && rm -rf /var/lib/apt/lists/*