# Basics of the Container Engine

The container engine, namely, Docker, is an open-source engine which allows you to create a lightweight, portable, and self-sufficient container for any application. SWR is compatible with Docker, allowing you to use Docker CLI and APIs to manage your images.

#### Installing Docker

Before installing Docker, get a basic understanding of what Docker is and how it works. For more information, see Docker Documentation.

Docker is compatible with almost all operating systems. Select a Docker version that best suits your needs. If you are not sure which Docker community edition to use, see https://docs.docker.com/engine/install/.

- Before using SWR to store container images, ensure that the Docker client used to push images to SWR is updated to 1.11.2 or later.
- Bind an elastic IP address first if your server runs in a private network as the installation requires Internet connection.

On a device running Linux, run the following commands to quickly install Docker:

    curl -fsSL get.docker.com -o get-docker.sh
    sh get-docker.sh
    sudo systemctl daemon-reload
    sudo systemctl restart docker

#### Building a Container Image

This section walks you through the steps of using a Dockerfile to build a container image for a simple web application. Dockerfile is a text file that contains all the instructions a user can call on the command line to build an image. A container image is a stack consisting of multiple layers. Each instruction creates a layer.

When using a browser to access a containerized application built from a Nginx image, you will see the default Nginx welcome page. In this section, you will build a new image based on the Nginx image to change the welcome message to Hello, SWR!

1. Log in to the device running Docker as a root user.
2. Run the following commands to create an empty file named Dockerfile:

   mkdir mynginx

   cd mynginx

   touch Dockerfile

3. Edit Dockerfile.

   vim Dockerfile

   Add the following instructions to the Dockerfile:

       FROM nginx
       RUN echo '<h1>Hello,SWR!</h1>' > /usr/share/nginx/html/index.html

   In the preceding instructions:

   - FROM: creates a layer from the base image. A valid Dockerfile must start with a FROM instruction. In this example, the Nginx image is used as the base image.
   - RUN: executes a command to create a new layer. One of its syntax forms is RUN <command>. In this example, the echo command is executed to display Hello, SWR!

   Save the changes and exit.

4. Run docker build [option] <context path> to build an image.

   docker build -t nginx:v1 .

   - -t nginx:v1: specifies the image name and tag.
   - .: indicates the path where the Dockerfile is located. All contents in this path are packed and sent to the Docker to build an image.
5. Run the following command to check the created image. The command output shows that the nginx image has been created with a tag of v1.

   docker images

#### Creating an Image Package

This section describes how to compress a container image into a .tar or .tar.gz package.

1. Log in to the device running Docker as a root user.
2. Run the following command to list images.

   docker images

   Check the name and tag of the image to be compressed.

3. Run the following command to compress the image into a package.

   docker save [OPTIONS] IMAGE [IMAGE...]

   OPTIONS: You can set this to --output or -o, indicating that the image is exported to a file.

   The file should be in either .tar or .tar.gz.

   Sample:

       $ docker save nginx:latest > nginx.tar
       $ ls -sh nginx.tar
       108M nginx.tar

       $ docker save php:5-apache > php.tar.gz
       $ ls -sh php.tar.gz
       372M php.tar.gz

       $ docker save --output nginx.tar nginx
       $ ls -sh nginx.tar
       108M nginx.tar

       $ docker save -o nginx-all.tar nginx
       $ docker save -o nginx-latest.tar nginx:latest