Debug Docker build steps

If a Docker build fails, you can inspect the intermediate steps.

The Rails asset compilation step below fails:

Step 10/12 : ADD . $APP_HOME
 ---> af4f399351dc
Step 11/12 : RUN bin/rake assets:precompile
 ---> Running in b5d9eec28028

We can run a command in the context of the last successful step:

docker run -it af4f399351dc sh