Add instructions for running one test in Docker (#2456)

Author: ttuegel

README.md

@@ -60,7 +60,8 @@ Use `docker.sh` to run commands inside the container:
 
 ``` sh
 ./docker/build.sh  # run once when dependencies change
-./docker/run.sh make ...
+./docker/run.sh make test  # run all tests
+./docker/run.sh make -C test/imp test  # run all tests in test/imp
 ```
 
 ### Recommended dependencies