3 Software installation simplified
three main ways to install Docker images:
- Docker Hub and other registries
- Using image files with docker save and dock load
- Building images with Dockerfiles
3.1 Identifying software
docker使用镜像来创建容器，镜像其实就是一个文件，这些文件包括用于创建容器的和关于镜像的元文件，这些元文件包括镜像间的关系，镜像的命令历史，暴露的端口，卷的定义等。
images都有唯一标识，原生的标示比较复杂。每次对镜像的修改都会使唯一标识改变，标识比较难用，我们使用仓库替代。
3.1.1 What is a repository
仓库就是存放镜像的地方，仓库的名字类似于URL。仓库名字由3个部分组成，image的位置、用户名、剪短的名字。
例如：
3.2 Finding and installing software
有许多的Docker indexes ,默认的是Docker Hub。Docker Hub is a registry and index with a website run by Docker Inc. （Docker Hub是一个注册表和索引，其中包含由Docker Inc运营的网站）。
3.2.1 Docker Hub from the command line
There are two ways that an image author can publish their images on Docker Hub:
- Use the command line to push images that they built independently and on their own sys-
tems. - Make a Dockerfile publicly available and use Docker Hub’s continuous build system.
docker search postgres
3.2.2 Docker Hub from the website
docker hub 网址
There are three other ways to install software:
- You can use alternative repository registries or run your own registry.
- You can manually load images from a file.
- You can download a project from some other source and build an image using a
provided Dockerfile.
3.2.2 Using alternative registries
使用一个可选的registry，不需要额外的配置，只需要列出resistry的地址。下面这个命令就是从一个制定的仓库中下载镜像的例子。
docker pull quay.io/dockerinaction/ch3_hello_registry:latest
有的时候，可能需要进行权限的认证，这要参考所选仓库的配置信息，进行权限认证。
The full pattern is as follows:
[REGISTRYHOST/][USERNAME/]NAME[:TAG]
3.2.4 Images as files
先拥有一个镜像，然后将镜像打包另存为文件，再删除进行，从文件中进行恢复。
docker pull busybox:latest
docker save -o myfile.tar busybox:latest
docker rmi busybox
docker load –i myfile.tar
You can use any filename you want. If you omit the –o flag,the resulting file will be streamed to the terminal.
