知识点3--首次使用Docker运行hello-world镜像

我们学习技术，对hello world这个单词肯定不陌生，无论是语言还是其他，只要是可控输出的，一般都是hello world，同样，我们学习doctor也从此开始，不过hello world镜像在阿里云里已经为我们准备好了一个，所以不需要我们自己去做这个镜像，我们只需要去运行run命令，让docker自己拉就行

docker run hello-world

run是Docker的容器运行命令，它的作用就相当于JAVA的new关键字一样，用镜像视为类，实例化出一个容器对象。我们可以通过日志看一下它干什么。

[root@hdp3 ~]# docker run hello-world
Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
2db29710123e: Pull complete 
Digest: sha256:2498fce14358aa50ead0cc6c19990fc6ff866ce72aeb5546e1d59caac3d0d60f
Status: Downloaded newer image for hello-world:latest

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
    (amd64)
 3. The Docker daemon created a new container from that image which runs the
    executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which sent it
    to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

在输出的日志中Unable to find image 'hello-world:latest' locally是告诉你docker现在本地仓库里没有找到hello-world这个镜像，且默认为最新版本。后面的Pull是告诉你在远程镜像库里面找到了该镜像，并开始下载，Digest和Status的输出告诉你下载结束并开始运行容器。

如果你输入了一个哪里都找不到的镜像，则返回失败。

[root@hdp3 ~]# docker run hello-worlddhuehd
Unable to find image 'hello-worlddhuehd:latest' locally
docker: Error response from daemon: pull access denied for hello-worlddhuehd, repository does not exist or may require 'docker login': denied: requested access to the resource is denied.
See 'docker run --help'.