Update Dockerfile base image to Debian Buster (RPi-Distro#295)

With the Qemu version shipped with Debian Stretch, the `man-db` package
being installed for Buster in the image triggers many of these errors:

    qemu: Unsupported syscall: 383

This is a manifestation of the following bug:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=891109

This is resolved in the current Qemu version shipped with Debian Buster.

Dockerfile

@@ -1,4 +1,4 @@
-FROM debian:stretch
+FROM debian:buster
 
 ENV DEBIAN_FRONTEND noninteractive