This assumes you have installed Docker on Linux and Docker is running.
A Docker image contains the code used to create a Docker container, such as creating a Nginx web server, or a mySQL server, or a home grown app, and the list goes on. In this way, an image is like a template used to create a container. An image is kind of like a virtual machine, but much more light weight, using significantly less storage a memory (containers are usually megabytes in size).
The docker cp command can be used to copy a file in a container to a directory on the host operating system, or to copy a file on the host operating system to a directory in the container.
You will want to first issue the docker ps or docker container ls command to get the list of containers. Let's say the docker ps command returns the following.
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES d937372c09ab9 b939aa938add9913 "/docker-entrypoin..." 6 minutes ago Created foo
Copy a single file from container to host
In this example, the /usr/local/bar.txt file in the "foo" container is copied to /tmp on the host operating system (the Docker system).
docker cp foo:/usr/local/bar.txt /tmp
Copy a directory from container to host
In this example, every file and directory below /usr/local in the foo container will be copied to /tmp on the host operating system (the Docker system).
docker cp foo:/usr/local /tmp
Copy from host to container
In this example, the /tmp/bar.txt file on the host operating system (the Docker system) is copied to /usr/local in the "foo" container.
docker cp /tmp/bar.txt foo:/usr/local