Upgrade to debian buster

Dockerfile

-FROM debian:stretch
+FROM debian:buster
 RUN apt-get update && apt-get install -y \
         adwaita-icon-theme \
         build-essential \

README.md

@@ -49,3 +49,13 @@ And then launch like this
 
 By default the cache will be 5GiB, you might want to increase it slightly by
 editing ccache.conf in the volume.
+
+## Upgrading to a new version of Debian
+
+If the Debian image gets too old, Qt will typically stop compiling after a
+while, since GCC stops getting updates and there will be compiler bugs that are
+not fixed in those old versions. To update the Debian version, change the first
+line in the `Dockerfile` with the codename for the new Debian release, i.e. for
+`buster`:
+
+    FROM debian:buster