Where is the package downloaded by docker?

Docker is a popular containerization tool that allows developers to more easily develop, test and deploy applications. When using Docker, you often need to download different images and packages to complete various tasks. So, where are the downloaded packages? This article will answer them one by one for you.

1. Docker image

Docker image is the basis on which the Docker container runs, and it is also the most commonly used package in Docker. Docker officially provides a mirror warehouse called Docker Hub, which contains rich mirror resources and also supports users to upload their own mirrors for other users to use.

When we need to download a Docker image, we can search and download it through Docker Hub. Use the following command to search for images:

docker search 镜像名

For example, if we want to search for an image named nginx, we can use the following command:

docker search nginx

Docker will return all images that meet the conditions. These The image has a corresponding page display on Docker Hub, which also contains the image version information and download link.

If you want to download this image, you can use the following command:

docker pull nginx

Docker will download the latest version of the nginx image. If you need to download a specified version of the nginx image, you can add the version number after the image name, for example:

docker pull nginx:1.18.0

After the download is completed, the image will be saved in the local Docker image warehouse. Use the following command to view the local image list:

docker images

2. Dockerfile

In addition to downloading ready-made Docker images, we can also write our own Dockerfile to build a custom image.

Dockerfile is a text file used to describe how a Docker image is built. In the Dockerfile, we can specify the required base image, install software packages, set environment variables, copy files and other operations. When Docker builds an image, it will execute it step by step according to the instructions in the Dockerfile, and finally generate a new Docker image.

When writing a Dockerfile, you can use a syntax similar to a shell script. Here is a simple example:

# 基础镜像
FROM ubuntu:20.04

# 设置环境变量
ENV APP_NAME=myapp

# 安装软件包
RUN apt update && \
    apt install -y nginx

# 复制文件
COPY index.html /var/www/html/

# 暴露端口
EXPOSE 80

# 设置启动命令
CMD ["nginx", "-g", "daemon off;"]

This Dockerfile specifies an image based on Ubuntu 20.04. It then installed the Nginx package and copied a file named index.html to Nginx's default root directory. Finally, it exposes port 80 and sets the startup command to run Nginx in daemon mode.

After writing the Dockerfile locally, we can use the following command to build the Docker image:

docker build -t 镜像名:版本号 .

In this command, -t is used to specify the name and version number of the image, and the last "." indicates the Dockerfile file in the current directory. After executing this command, Docker will build a new Docker image according to the instructions in the Dockerfile and save it in the local image warehouse.

3. Container internal files

In addition to the Docker image and Dockerfile, we can also obtain the downloaded package through the files inside the container. When we start a Docker container, it generates a runtime instance from the image, and we can perform various operations in this runtime instance.

For example, we can start a container to run Nginx, and mount a directory on the host to the container to store the Nginx configuration file. Inside the container, we can use the following command to find the downloaded Nginx software package:

find / -name nginx

With this command, we can find the directory where the Nginx software package is located, and then obtain the full path of the software package. Of course, this approach only works if we manually downloaded the package inside the container.

Summary

The packages downloaded by Docker are usually saved in three places: the official image warehouse, the local image warehouse and inside the container. We can search and download ready-made images through Docker Hub, or we can custom-generate images through Dockerfile. Inside the container, we can manually download the required software packages, or we can view the files inside the image to obtain the downloaded packages. These methods can help us better utilize Docker for application development and deployment.

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