Switch to Buster

Dockerfile

@@ -2,7 +2,7 @@ FROM debian:stable AS build
 ADD . /src
 RUN apt-get -q update && env DEBIAN_FRONTEND=noninteractive apt-get -qy install --no-install-recommends rsync git golang ca-certificates && cd /src && ./scripts/lint.sh && ./scripts/update.sh
 
-FROM registry.git.autistici.org/ai3/docker/apache2-base:master
+FROM registry.git.autistici.org/ai3/docker/apache2-base:buster
 
 COPY --from=build /src/public /var/www/autistici.org
 COPY --from=build /src/build/bin/sitesearch /usr/sbin/sitesearch