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dockerclient来构建的image。
dockerclient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--outputaspnetcore.tar.gzhttps://dotnetcli.azureedge.net/dotnet/aspnetcore/Runtime/$ASPNETCORE_VERSION/aspnetcore-runtime-$ASPNETCORE_VERSION-linux-x64.tar.gz&&aspnetcore_sha512='954072376698be69acb7e277df2c243f931e10529def21dcbf9ce277609b30d462126bf8b8b3cab36476bec3d63a927b8e44e59e4d4cade23eef45956fba1ffd'&&echo\"$aspnetcore_sha512aspnetcore.tar.gz\"|sha512sum-c-&&mkdir-p/usr/share/dotnet&&tar-zxfaspnetcore.tar.gz-C/usr/share/dotnet&&rmaspnetcore.tar.gz&&ln-s/usr/share/dotnet/dotnet/usr/bin/dotnet"
],
"Image":"sha256:5ecfe4016ac8e911a94aa601a675f7204e9ccab00cbb08e7067c184ad40f34e9",
"Volumes":null,
"WorkingDir":"",
"Entrypoint":null,
"OnBuild":null,
"Labels":null
},
"created":"2019-12-28T08:12:05.676492579Z",
"docker_version":"3.0.8",
"history":[
{
"created":"2019-12-28T04:23:47.4966447Z",
"created_by":"/bin/sh-c#(nop)ADDfile:90a2c81769a336bed3f731f44a385f2a65b0916f517a0b77c06c224579bf9a9ain/"
},
{
"created":"2019-12-28T04:23:47.719507596Z",
"created_by":"/bin/sh-c#(nop)CMD[\"bash\"]",
"empty_layer":true
},
{
"created":"2019-12-28T08:11:05.607009582Z",
"created_by":"/bin/sh-capt-getupdate&&apt-getinstall-y--no-install-recommendsca-certificateslibc6libgcc1libgssapi-krb5-2libicu57liblttng-ust0libssl1.0.2libstdc++6zlib1g&&rm-rf/var/lib/apt/lists/*"
},
{
"created":"2019-12-28T08:11:07.64336022Z",
"created_by":"/bin/sh-c#(nop)ENVASPNETCORE_URLS=http://+:80DOTNET_RUNNING_IN_CONTAINER=true",
"empty_layer":true
},
{
"created":"2019-12-28T08:11:16.475068844Z",
"created_by":"/bin/sh-capt-getupdate&&apt-getinstall-y--no-install-recommendscurl&&rm-rf/var/lib/apt/lists/*"
},
{
"created":"2019-12-28T08:11:43.814078508Z",
"created_by":"/bin/sh-c#(nop)ENVASPNETCORE_VERSION=2.2.8",
"empty_layer":true
},
{
"created":"2019-12-28T08:12:05.676492579Z",
"created_by":"/bin/sh-ccurl-SL--outputaspnetcore.tar.gzhttps://dotnetcli.azureedge.net/dotnet/aspnetcore/Runtime/$ASPNETCORE_VERSION/aspnetcore-runtime-$ASPNETCORE_VERSION-linux-x64.tar.gz&&aspnetcore_sha512='954072376698be69acb7e277df2c243f931e10529def21dcbf9ce277609b30d462126bf8b8b3cab36476bec3d63a927b8e44e59e4d4cade23eef45956fba1ffd'&&echo\"$aspnetcore_sha512aspnetcore.tar.gz\"|sha512sum-c-&&mkdir-p/usr/share/dotnet&&tar-zxfaspnetcore.tar.gz-C/usr/share/dotnet&&rmaspnetcore.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
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
- dockerhub
- aliyun
- 腾讯云
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