Docker 镜像刨析

Docker镜像是什么

镜像是一种轻量级的可执行的独立的软件包,用来打包软件运行环境和基于运行环境开发的软件,它包含运行软件所需要的所有内容,包括代码,运行时库,环境变量和配置文件;

举个例子:

我们要安装tomcat 大约11M

​​去官网下载tomcat​​

我们在docker上下载的tomcat镜像—483M

这是因为tomcat运行所需要jdk环境,所以在下载镜像时会将tomcat所需要的依赖(运行是库等)一起打包下载下来;

tomcat镜像是由三层结构构成的;各个镜像之间可以共用某些资源;比如centos的一些镜像

下载第二个镜像时速度会非常快(其实不算是从仓库下载,而是本地的base镜像复制一份,运行时在内存中再加载一份)

为了更直观的查看镜像的分层信息----执行下面的命令:

docker image inspect tomcat:latest

执行之后---->>

[
    {
        "Id": "sha256:451d25ef4583d58da6cb84bbbfd36aa6e752a8efa3d9a137da9b77334e7b902b",
        "RepoTags": [
            "tomcat:latest"
        ],
        "RepoDigests": [
            "tomcat@sha256:9edc5c51c2dec7794769eede1e88a60e77abe355b73a77e231c464a8384d3697"
        ],
        "Parent": "",
        "Comment": "",
        "Created": "2022-06-28T00:54:54.364194878Z",
        "Container": "049353471f080c6f9cdb481b1e99cf40d263c1130a657161c9c2f064ec557c05",
        "ContainerConfig": {
            "Hostname": "049353471f08",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "ExposedPorts": {
                "8080/tcp": {}
            },
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/tomcat/bin:/opt/java/openjdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "LANG=en_US.UTF-8",
                "LANGUAGE=en_US:en",
                "LC_ALL=en_US.UTF-8",
                "JAVA_VERSION=jdk-17.0.3+7",
                "JAVA_HOME=/opt/java/openjdk",
                "CATALINA_HOME=/usr/local/tomcat",
                "TOMCAT_NATIVE_LIBDIR=/usr/local/tomcat/native-jni-lib",
                "LD_LIBRARY_PATH=/usr/local/tomcat/native-jni-lib",
                "GPG_KEYS=A9C5DF4D22E99998D9875A5110C01C5A2F6059E7",
                "TOMCAT_MAJOR=10",
                "TOMCAT_VERSION=10.0.22",
                "TOMCAT_SHA512=fe46db8794f066882b30e7a94bd8d3dbcf29e8e8ffaf67c1355846755745a7c9eafd124819283f218bcf410921a485b44b57b56fd6251fb99d67d95f3dd36826"
            ],
            "Cmd": [
                "/bin/sh",
                "-c",
                "#(nop) ",
                "CMD [\"catalina.sh\" \"run\"]"
            ],
            "Image": "sha256:fa0b50a395becf0a0a5c020fe2e2e2b46a59ed714c9543cf44bc45c6fd28c4e9",
            "Volumes": null,
            "WorkingDir": "/usr/local/tomcat",
            "Entrypoint": null,
            "OnBuild": null,
            "Labels": {}
        },
        "DockerVersion": "20.10.12",
        "Author": "",
        "Config": {
            "Hostname": "",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "ExposedPorts": {
                "8080/tcp": {}
            },
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/tomcat/bin:/opt/java/openjdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "LANG=en_US.UTF-8",
                "LANGUAGE=en_US:en",
                "LC_ALL=en_US.UTF-8",
                "JAVA_VERSION=jdk-17.0.3+7",
                "JAVA_HOME=/opt/java/openjdk",
                "CATALINA_HOME=/usr/local/tomcat",
                "TOMCAT_NATIVE_LIBDIR=/usr/local/tomcat/native-jni-lib",
                "LD_LIBRARY_PATH=/usr/local/tomcat/native-jni-lib",
                "GPG_KEYS=A9C5DF4D22E99998D9875A5110C01C5A2F6059E7",
                "TOMCAT_MAJOR=10",
                "TOMCAT_VERSION=10.0.22",
                "TOMCAT_SHA512=fe46db8794f066882b30e7a94bd8d3dbcf29e8e8ffaf67c1355846755745a7c9eafd124819283f218bcf410921a485b44b57b56fd6251fb99d67d95f3dd36826"
            ],
            "Cmd": [
                "catalina.sh",
                "run"
            ],
            "Image": "sha256:fa0b50a395becf0a0a5c020fe2e2e2b46a59ed714c9543cf44bc45c6fd28c4e9",
            "Volumes": null,
            "WorkingDir": "/usr/local/tomcat",
            "Entrypoint": null,
            "OnBuild": null,
            "Labels": null
        },
        "Architecture": "amd64",
        "Os": "linux",
        "Size": 482519550,
        "VirtualSize": 482519550,
        "GraphDriver": {
            "Data": {
                "LowerDir": "/var/lib/docker/overlay2/7528c10792622ebe441a52fd0b6e4e3315876efef47156aaa194db29a52d2a49/diff:/var/lib/docker/overlay2/a80446b459c17627a18b6a1c50c118324046c5eeef52e34ef8bff11904da9cad/diff:/var/lib/docker/overlay2/a75c8fa300341c107196edc36d4f9ccd0c41686bcdc957d07e1be724a5e63ef4/diff:/var/lib/docker/overlay2/d2205f56c88b01207462b5ab463231d75bd73fe28f283bd544d347afd532d14f/diff:/var/lib/docker/overlay2/37d44d9e0afaca2f3f59953a7fadc1e19a33328c2591e724d7d445f8f319e204/diff:/var/lib/docker/overlay2/07b640eac09047b7edbf33a16b5f59eaf813f7fb8ce3e6d6bdc6d3c2a64c1ae9/diff",
                "MergedDir": "/var/lib/docker/overlay2/3a0d1078504f3de9f66dcdfeed164eee8dbf28a3a099a5a0f17ccd764f99a02d/merged",
                "UpperDir": "/var/lib/docker/overlay2/3a0d1078504f3de9f66dcdfeed164eee8dbf28a3a099a5a0f17ccd764f99a02d/diff",
                "WorkDir": "/var/lib/docker/overlay2/3a0d1078504f3de9f66dcdfeed164eee8dbf28a3a099a5a0f17ccd764f99a02d/work"
            },
            "Name": "overlay2"
        },
        "RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:a790f937a6aea2600982de54a5fb995c681dd74f26968d6b74286e06839e4fb3",
                "sha256:16f636807506521a66f09cdd9eaff9b911004f0284e400a04f70fe184006569b",
                "sha256:e369ffcb8f5d3ec8be94988ccec7f99d36b6f5eeb197421bf7512cb2bb761ae0",
                "sha256:571824725172feb66b22770b3902a40c7bc2d6285eff9d4e5804bf4619fe776f",
                "sha256:31c135ba5e112ab8d4c40cb2694952a8021fda4fe76db3ba6e1efa67f23163e3",
                "sha256:6171d86da26955eb21b3fa0671a31bb6c4cdfaa44b4fde339ea530a3bb899f41",
                "sha256:e019a18df1faf83dc044ac7b1839df0b3b64ef19fe9de1f17f10b5886790c156"
            ]
        },
        "Metadata": {
            "LastTagTime": "0001-01-01T00:00:00Z"
        }
    }
]

上面提到了镜像其实是一个分层结构,分层结构的好处就是提高了复用性----这里就要提到UnionFS;

UnionFS（联合文件系统）: Union文件系统（UnionFS）是一种分层、轻量级并且高性能的文件系统，它支持对文件系统的修改作为一次提交来一层层的叠加，同时可以将不同目录挂载到同一个虚拟文件系统下(unite several directories into a single virtual filesystem)。

Union 文件系统是 Docker 镜像
的基础。镜像可以通过分层来进行继承，基于基础镜像（没有父镜像），可以制作各种具体的应用镜像。

联合文件系统的特性:
一次同时加载多个文件系统，但从外面看起来，只能看到一个;

镜像的特点:

Docker镜像都是只读的,当启动一个镜像时,一个新的可写的层会被加载到镜像层顶部,这一层就是我们所说的容器层,容器层之下的叫做镜像层;

分层体现:

镜像的操作,有时候我们写好了一个程序,把这个程序做成对应的镜像,以便后续继续使用,这时候我们就需要docker commit 命令了

首先我们尝试运行一个容器—tomcat

docker run -it -p 8888:8080 tomcat

运行之后访问 8888端口----404错误

我们为什么会访问404错误呢?

我们进入到容器里面看一下

# 看一下i容器的 id
C:\Users\Gavin>docker ps
CONTAINER ID   IMAGE     COMMAND             CREATED       STATUS       PORTS                    NAMES
961c9b239a6c   tomcat    "catalina.sh run"   5 hours ago   Up 5 hours   0.0.0.0:8888->8080/tcp   vibrant_satoshi

之后进入容器里面,我们知道访问tomcat时会,我们会将一些资源文件放到webapp里面

进入tomcat这个容器,

这里有两个命令需要区分一些
docker attach

exec 在容器中打开新的终端,并且可以启动新的进程
attach 直接进入容器启动命令的终端，不会启动新的进程

docker attach containerid 进入容器

发现进不去,

那就换一种方式试一下

dockcer exec -it containerid /bin/bash

进入webapps里面,发现什么都没有

接下来就是向docker容器里面添加一些文件了;

执行 docker cp 命令

C:\Users\Gavin>docker exec -it 961c /bin/bash  #进入容器
root@961c9b239a6c:/usr/local/tomcat# ls
bin              lib             NOTICE         temp
BUILDING.txt     LICENSE         README.md      webapps
conf             logs            RELEASE-NOTES  webapps.dist
CONTRIBUTING.md  native-jni-lib  RUNNING.txt    work
root@961c9b239a6c:/usr/local/tomcat/webapps #查看webapps里面的文件 
C:\Users\Gavin>docker cp d:/ROOT  961c:/usr/local/tomcat/webapps/ROOT  #从宿主机复制文件到 容器

C:\Users\Gavin>docker exec -it 961c /bin/bash
root@961c9b239a6c:/usr/local/tomcat# cd webapps
root@961c9b239a6c:/usr/local/tomcat/webapps# ls
ROOT
root@961c9b239a6c:/usr/local/tomcat/webapps# cd ROOT/
root@961c9b239a6c:/usr/local/tomcat/webapps/ROOT# ls
asf-logo-wide.svg  bg-nav.png    index.jsp          tomcat.svg
bg-button.png      bg-upper.png  RELEASE-NOTES.txt  WEB-INF
bg-middle.png      favicon.ico   tomcat.css

接着重新运行该容器

docker restart containerid

C:\Users\Gavin>docker ps
CONTAINER ID   IMAGE     COMMAND             CREATED       STATUS       PORTS                    NAMES
961c9b239a6c   tomcat    "catalina.sh run"   5 hours ago   Up 5 hours   0.0.0.0:8888->8080/tcp   vibrant_satoshi

C:\Users\Gavin>docker restart  961c
961c

C:\Users\Gavin>docker ps
CONTAINER ID   IMAGE     COMMAND             CREATED       STATUS         PORTS                    NAMES
961c9b239a6c   tomcat    "catalina.sh run"   5 hours ago   Up 7 seconds   0.0.0.0:8888->8080/tcp   vibrant_satoshi

查看对应端口,发现已经有了相应访问内容;

所以当我们pull下载相应镜像时,一些文件并没有,所以我们要自己打包一下镜像以便下次继续访问;

制作镜像:

下面自己制作镜像文件:

docker commit

C:\Users\Gavin>docker commit -a"gavin" -m"add tomcat necefile" 961c gav
in/tomcat1.1
sha256:0e1da64fe76ea8ec2365530542b87fb3e160c41d4f1a1f1780a0bd3153b84c8b

命令解析:

C:\Users\Gavin>docker commit --help

Usage:  docker commit [OPTIONS] CONTAINER [REPOSITORY[:TAG]]

Create a new image from a container's changes

Options:
  -a, --author string    Author (e.g., "John Hannibal Smith
                         <hannibal@a-team.com>")  #作者信息
  -c, --change list      Apply Dockerfile instruction to the created image   
  -m, --message string   Commit message  #镜像信息 
  -p, --pause            Pause container during commit  #(default true) 在提交期间暂停容器

查看制作的容器----->>

运行镜像

执行制作好的tomcat容器

查看制作的镜像:

运行镜像---->>

C:\Users\Gavin>docker run -it -p 8888:8080 gavin/tomcat1.1
Using CATALINA_BASE:   /usr/local/tomcat
Using CATALINA_HOME:   /usr/local/tomcat
Using CATALINA_TMPDIR: /usr/local/tomcat/temp
Using JRE_HOME:        /opt/java/openjdk
Using CLASSPATH:       /usr/local/tomcat/bin/bootstrap.jar:/usr/local/tomcat/bin/tomcat-juli.jar