For copying the files from the host to the docker container :

Step1: List out the container,

$ docker container ls
CONTAINER ID   IMAGE              COMMAND                  CREATED          STATUS          PORTS                                     NAMES
74a4b83f1bb7   wordpress:latest   "docker-entrypoint.s…"   26 seconds ago   Up 24 seconds>80/tcp, :::31010->80/tcp   thelinuxfaq_1

First, we have to set the path in our localhost to where the file is stored in a docker container, 

Next set the path in our docker container to where we want to store the file inside our docker container, here the container name is thelinuxfaq_1.

Then copy the file which we want to store in our docker container with the help of CP command or you're able to get the details using the command below,

$ docker --help
  cp          Copy files/folders between a container and the local filesystem

docker cp  [Local_file]  [CONTAINER_NAME]:[DESTINATION]

$ docker cp /home/user/newfile.txt  thelinuxfaq_1:/app/newfile.txt

For copying Files from docker container to your host:

We can use the same "CP" option in docker,

docker cp [CONTAINER_NAME]:[SOURCE_FILE]  [Local_Host]

$ docker cp thelinuxfaq_1:/app/newfile.txt /home/user/newfile.txt