step 2: build docker image

1. Create Dockerfile

   ref:https://docs.docker.com/engine/reference/builder/

2. Build docker image

    $ docker build -t myapp:0.1 .
    $ #or 
    $ docker pull nginx:latest

3. save docker image to tar file

    $ docker images
    REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
    nginx               latest              f68d6e55e065        9 hours ago         109MB
    $ docker save -o mynginx.tar nginx