docker: Add python3 to the docker image

Most user-contributed plugins are written in python, and not having it
available in the docker image is rather annoying to work around. So we just
add that tiny dependency to the image.

Fixes #3765

Dockerfile

@@ -82,7 +82,7 @@ RUN ./configure --prefix=/tmp/lightning_install --enable-static && make -j3 DEVE
 
 FROM debian:stretch-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 \
+RUN apt-get update && apt-get install -y --no-install-recommends socat inotify-tools python3 python3-pip \
     && rm -rf /var/lib/apt/lists/*
 
 ENV LIGHTNINGD_DATA=/root/.lightning