Merge branch 'buster' into 'master'

Upgrade base images to buster

Closes #1

See merge request !1

Dockerfile

 # Stage 1: build the tiny smtp server
-FROM golang:1.12.9 AS build
+FROM golang:latest AS build
 WORKDIR /go/src
 RUN go get -d github.com/chrj/smtpd
 COPY smtpd.go .
 RUN go build -o smtpd smtpd.go
 
 # Stage 2: build the final container
-FROM registry.git.autistici.org/ai3/docker/apache2-base:master
+FROM registry.git.autistici.org/ai3/docker/apache2-base:buster
 COPY start.sh /start.sh
 COPY build.sh /tmp/build.sh
 COPY ai-lurker-ui.patch /tmp/ai-lurker-ui.patch