The main problem with docker on debugging has to do with images that already have the 
CMD command specified.
If something goes wrong and has to do with the filesystem, or some commands that should have taken effect and they did not you need to do some troubleshooting.
Overriding the command which is executed should be helpful. I do this all the time on custom images since I need some bash in order to be able to troubleshoot.
Supposing I need to troubleshoot something on the default nginx image.
Then nginx image should run like this.
docker run --rm nginx
Now instead of running our server, we shall override the command with a bash session (enter a shell by executing /bin/sh).
docker run --rm -it --entrypoint "/bin/sh" nginx
If you also want to pass arguments to the executable specified there is also another workaround.
docker run --rm -it --entrypoint "ls" nginx -l
Emmanouil Gkatziouras 