Update README.md

Author: partouf

README.md

@@ -2,3 +2,17 @@
 
 The repository is part of the [Compiler Explorer](https://godbolt.org/) project. It builds
 the docker images used to build the various GCC compilers used on the site.
+
+## To Test
+
+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 -t gccbuilder .`
+* `docker run gccbuilder ./build.sh trunk`
+
+### Alternative to run (for better debugging)
+
+* `docker run -t -i gccbuilder bash`
+* `./build.sh trunk`