Clarify readme

Author: mattgodbolt

README.md

@@ -9,9 +9,12 @@ This assumes you have set up your user account to be able to run
 `docker` [without being root](https://docs.docker.com/engine/security/rootless/);
 if you haven't done so, you'll need to prefix these commands with `sudo`.
 
-* `docker build --build-arg image=22.04 -t clangbuilder .`
+* `docker build -t clangbuilder .`
 * `docker run clangbuilder ./build.sh trunk`
 
+If you need to change the default Ubuntu image, add `--build-arg image=20.04`
+or similar to the build step.
+
 ### Alternative to run (for better debugging)
 
 * `docker run -t -i clangbuilder bash`