These commands will come in handy at some point in life:

# Stop running containers, then delete all containers:
docker stop $(docker ps -q) && docker rm $(docekr ps -a -q)

# Delete all images:
docker rmi --force $(docker images -a -q)

Explanation: The -q|--quiet flag will output just the container IDs or image IDs. -a|--all will list out all containers, running or otherwise, or list out all images including intermediary layers.