docker: base our image on 10-buster-slim instead of buster-slim

A Docker base image without version is a bit of a moving target. Buster-slim,
for example, is currently based on nodejs 12.

For now, let's base our official Docker image on nodejs 10 (an LTS, non at End
of Life, which we explicitly mention in the documentation).

Amends a9a3bf9bd2 and the corresponding PR #3646.

docker/Dockerfile

@@ -6,7 +6,7 @@
 #
 # Version 0.1
 
-FROM node:buster-slim
+FROM node:10-buster-slim
 LABEL maintainer="Etherpad team, https://github.com/ether/etherpad-lite"
 
 # git hash of the version to be built.