Docker镜像构建原理解析(不装docker也能构建镜像)

在devops流程里面构建镜像是一个非常重要的过程，一般构建镜像是写dockerfile文件然后通过docker client来构建的image。

docker client 会先检查本地有没有image，如果没有帮你从镜像仓库 pull 下来

然后解析你写的dockerfile构建新的image。

本文带你了解

pull 命令背后是怎么做的？
build 命令背后是怎么做的？

下篇文章带你解析：

如果我不用docker 我如何构建一个镜像？

我们以微软的aspnet2.2为基础构建一个aspnetcore项目的镜像为例子

mcr.microsoft.com/dotnet/core/aspnet:2.2

根据基础镜像REGISTRY去获取mainfest信息

https://mcr.microsoft.com/v2/dotnet/core/aspnet/manifests/2.2

Accept:

application/vnd.oci.image.manifest.v1+json,application/vnd.docker.distribution.manifest.v2+json,application/vnd.docker.distribution.manifest.v1+json

获取到的内容如下：

{

    "schemaVersion": 2,

    "mediaType": "application/vnd.docker.distribution.manifest.v2+json",

    "config": {

        "mediaType": "application/vnd.docker.container.image.v1+json",

        "size": 4039,

        "digest": "sha256:e7e3b238011ce0f2b9350153535fe273caa01f0e7188d0b91f965b3802ddc600"

    },

    "layers": [

        {

            "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",

            "size": 22524609,

            "digest": "sha256:804555ee037604c40de144f9f8da0d826d38db82f15d74cded32790fe279a8f6"

        },

        {

            "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",

            "size": 17692725,

            "digest": "sha256:970251047358aea56ba6db6975b14ff12470b75de0c2477f4445240ddd727fd4"

        },

        {

            "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",

            "size": 2978257,

            "digest": "sha256:f3d4c41a4fd13f35c0b46f19a4e27845f4695163cc7174d908ff84836bbc2f5a"

        },

        {

            "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",

            "size": 62145592,

            "digest": "sha256:bd391c46585f9f8d84992bbaa9087189148c1601968eaaf097d5b3ed60840e5e"

        }

    ]

}

mainfest文件里面都是摘要(digest)记录

config信息摘要
每个layer的摘要（上面的例子有4个）

根据上面的config信息摘要获取config详情

GET:https://mcr.microsoft.com/v2/dotnet/core/aspnet/blobs/sha256:e7e3b238011ce0f2b9350153535fe273caa01f0e7188d0b91f965b3802ddc600



{

    "architecture": "amd64",

    "config": {

        "Hostname": "",

        "Domainname": "",

        "User": "",

        "AttachStdin": false,

        "AttachStdout": false,

        "AttachStderr": false,

        "Tty": false,

        "OpenStdin": false,

        "StdinOnce": false,

        "Env": [

            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",

            "ASPNETCORE_URLS=http://+:80",

            "DOTNET_RUNNING_IN_CONTAINER=true",

            "ASPNETCORE_VERSION=2.2.8"

        ],

        "Cmd": [

            "bash"

        ],

        "ArgsEscaped": true,

        "Image": "sha256:5ecfe4016ac8e911a94aa601a675f7204e9ccab00cbb08e7067c184ad40f34e9",

        "Volumes": null,

        "WorkingDir": "",

        "Entrypoint": null,

        "OnBuild": null,

        "Labels": null

    },

    "container": "14196c2f9c327d41e26682d32c7c89c4e7c78aa32f8b7501a23192035a9f4844",

    "container_config": {

        "Hostname": "",

        "Domainname": "",

        "User": "",

        "AttachStdin": false,

        "AttachStdout": false,

        "AttachStderr": false,

        "Tty": false,

        "OpenStdin": false,

        "StdinOnce": false,

        "Env": [

            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",

            "ASPNETCORE_URLS=http://+:80",

            "DOTNET_RUNNING_IN_CONTAINER=true",

            "ASPNETCORE_VERSION=2.2.8"

        ],

        "Cmd": [

            "/bin/sh",

            "-c",

            "curl -SL --output aspnetcore.tar.gz https://dotnetcli.azureedge.net/dotnet/aspnetcore/Runtime/$ASPNETCORE_VERSION/aspnetcore-runtime-$ASPNETCORE_VERSION-linux-x64.tar.gz     && aspnetcore_sha512='954072376698be69acb7e277df2c243f931e10529def21dcbf9ce277609b30d462126bf8b8b3cab36476bec3d63a927b8e44e59e4d4cade23eef45956fba1ffd'     && echo \"$aspnetcore_sha512  aspnetcore.tar.gz\" | sha512sum -c -     && mkdir -p /usr/share/dotnet     && tar -zxf aspnetcore.tar.gz -C /usr/share/dotnet     && rm aspnetcore.tar.gz     && ln -s /usr/share/dotnet/dotnet /usr/bin/dotnet"

        ],

        "Image": "sha256:5ecfe4016ac8e911a94aa601a675f7204e9ccab00cbb08e7067c184ad40f34e9",

        "Volumes": null,

        "WorkingDir": "",

        "Entrypoint": null,

        "OnBuild": null,

        "Labels": null

    },

    "created": "2019-12-28T08:12:05.676492579Z",

    "docker_version": "3.0.8",

    "history": [

        {

            "created": "2019-12-28T04:23:47.4966447Z",

            "created_by": "/bin/sh -c #(nop) ADD file:90a2c81769a336bed3f731f44a385f2a65b0916f517a0b77c06c224579bf9a9a in / "

        },

        {

            "created": "2019-12-28T04:23:47.719507596Z",

            "created_by": "/bin/sh -c #(nop)  CMD [\"bash\"]",

            "empty_layer": true

        },

        {

            "created": "2019-12-28T08:11:05.607009582Z",

            "created_by": "/bin/sh -c apt-get update     && apt-get install -y --no-install-recommends         ca-certificates                 libc6         libgcc1         libgssapi-krb5-2         libicu57         liblttng-ust0         libssl1.0.2         libstdc++6         zlib1g     && rm -rf /var/lib/apt/lists/*"

        },

        {

            "created": "2019-12-28T08:11:07.64336022Z",

            "created_by": "/bin/sh -c #(nop)  ENV ASPNETCORE_URLS=http://+:80 DOTNET_RUNNING_IN_CONTAINER=true",

            "empty_layer": true

        },

        {

            "created": "2019-12-28T08:11:16.475068844Z",

            "created_by": "/bin/sh -c apt-get update     && apt-get install -y --no-install-recommends         curl     && rm -rf /var/lib/apt/lists/*"

        },

        {

            "created": "2019-12-28T08:11:43.814078508Z",

            "created_by": "/bin/sh -c #(nop)  ENV ASPNETCORE_VERSION=2.2.8",

            "empty_layer": true

        },

        {

            "created": "2019-12-28T08:12:05.676492579Z",

            "created_by": "/bin/sh -c curl -SL --output aspnetcore.tar.gz https://dotnetcli.azureedge.net/dotnet/aspnetcore/Runtime/$ASPNETCORE_VERSION/aspnetcore-runtime-$ASPNETCORE_VERSION-linux-x64.tar.gz     && aspnetcore_sha512='954072376698be69acb7e277df2c243f931e10529def21dcbf9ce277609b30d462126bf8b8b3cab36476bec3d63a927b8e44e59e4d4cade23eef45956fba1ffd'     && echo \"$aspnetcore_sha512  aspnetcore.tar.gz\" | sha512sum -c -     && mkdir -p /usr/share/dotnet     && tar -zxf aspnetcore.tar.gz -C /usr/share/dotnet     && rm aspnetcore.tar.gz     && ln -s /usr/share/dotnet/dotnet /usr/bin/dotnet"

        }

    ],

    "os": "linux",

    "rootfs": {

        "type": "layers",

        "diff_ids": [

            "sha256:814c70fdae62bc26c603bfae861f00fb1c77fc0b1ee8d565717846f4df24ae5d",

            "sha256:0cf75cb98eb2e0a82631d4aff71b40ba79ff7f83e0361f696875e592a1a4cefc",

            "sha256:15e45d99c92686fb1fd61a41431d8400d7a0e8381595d09d666b0809c4f5d993",

            "sha256:579a8f1d6a123f98095c0b1a1395079f7504391fd2a8bc529dede305a2072a36"

        ]

    }

}

根据diff_ids里面去下载对应layers

下载完后对比摘要一致，确保镜像文件合法性

路径规则：

https://mcr.microsoft.com/v2/dotnet/core/aspnet/blobs/sha256:XXXXXX

构建我们的镜像

在基础镜像的配置基础上加入我们的自定义配置

Entrypoint
Cmd
Ports
Environment
ImageWorkingDirectory
Volumes
Labels

在基础镜像的所有的layers把我们要打包到镜像也做成一个layer

生成的镜像tar包解压出来

多了一个 tar.gz文件，解压之后就是我们打包放进去的文件

原来基础镜像有4个layer 加上我们的共5个
config.json
manifest.json

manifest.json对比与基础镜像

config.json对比与基础镜像

我们来复习下构建镜像的过程

根据镜像名称拉取mainfest

根据mainfest拉取config

根据config拉取layers

下载各个layer

修改到基础镜像的配置(config.json和mainfest.json)

加入我们要加入的文件layer

知道原理后我们可以自己写一个工具来实现整个过程

我开源的docker镜像构建，让你不用装docker也能快速构建容器镜像

支持以下镜像仓库作为基础镜像构建

docker hub
aliyun
腾讯云

Docker镜像构建原理解析(不装docker也能构建镜像)

根据基础镜像REGISTRY去获取mainfest信息

根据上面的config信息摘要获取config详情

根据diff_ids里面去下载对应layers

构建我们的镜像

生成的镜像tar包解压出来

manifest.json对比与基础镜像

config.json对比与基础镜像

我们来复习下构建镜像的过程

知道原理后我们可以自己写一个工具来实现整个过程

Docker镜像构建原理解析(不装docker也能构建镜像)的相关教程结束。

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