无论是在我们的生产环境或测试环境中，一旦涉及到docker pull和push的时候，我们都迫切希望Docker镜像是一个非常小的file，一方面在网络带宽有限的情况下，image的size越小下载所付出的时间代价就越小，另一方面image始终是一个文件，size对存储空间是有一定影响的，看来这是个提高生产力的问题，那么我们如何去构建一个size很小的image呢？
本文内容
- 单阶段构建镜像
- 多阶段构建镜像
- 更小的镜像构建
注：多阶段构建是 Docker 17.05 及更高版本提供的功能
查看一下image列表：
$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
golang 1.10.3 d0e7a411e3da 6 weeks ago 794MB
alpine 3.8 11cd0b38bc3c 8 weeks ago 4.41MB通常在我们什么都不考虑的情况下，构建一个golang的应用镜像是非常简单的，只需要pull官方的golang环境，把我们的程序copy进去就可以了，下面我们先创建一个工程的目录如下：
$ tree -L 2 -C
.
├── Dockerfile
└── src
└── main.go
1 directory, 2 files单阶段构建镜像
- Dockerfile:
FROM golang:1.10.3
WORKDIR /go/src/test
RUN go get github.com/gin-gonic/gin
COPY src src
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main src/main.go
CMD ["./main"]- Build
$ docker build -t zev/test:1.0.0 .
Sending build context to Docker daemon 17.41kB
Step 1/6 : FROM golang:1.10.3
---> d0e7a411e3da
Step 2/6 : WORKDIR /go/src/test
---> Running in 94d1ede51e17
Removing intermediate container 94d1ede51e17
---> 2b643ce8b3cf
Step 3/6 : RUN go get github.com/gin-gonic/gin
---> Running in de5e9adb7c10
Removing intermediate container de5e9adb7c10
---> ff970f45de1e
Step 4/6 : COPY src src
---> 6b79fef06e45
Step 5/6 : RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main src/main.go
---> Running in 6d4ef8c0b580
Removing intermediate container 6d4ef8c0b580
---> 59678a3ab4d8
Step 6/6 : CMD ["./main"]
---> Running in a5cea54f2ccb
Removing intermediate container a5cea54f2ccb
---> a253cfcddd6a
Successfully built a253cfcddd6a
Successfully tagged zev/test:1.0.0- RUN
$ docker run -it -p 8080:8080 zev/test:1.0.0
[GIN-debug] [WARNING] Now Gin requires Go 1.6 or later and Go 1.7 will be required soon.
[GIN-debug] [WARNING] Creating an Engine instance with the Logger and Recovery middleware already attached.
[GIN-debug] [WARNING] Running in "debug" mode. Switch to "release" mode in production.
- using env: export GIN_MODE=release
- using code: gin.SetMode(gin.ReleaseMode)
[GIN-debug] GET /ping --> main.main.func1 (3 handlers)
[GIN-debug] Listening and serving HTTP on :8080- Images
$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
zev/test 1.0.0 a253cfcddd6a 4 minutes ago 857MBimage的size为857MB，内部包含了整个golang环境，这么大的文件在传输中绝对是个灾难，接下来我们用多阶段构建一个相对比较小的image。
多阶段构建镜像
- Dockerfile:
FROM golang:1.10.3 as builder
WORKDIR /go/src/test
RUN go get github.com/gin-gonic/gin
COPY src src
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main src/main.go
FROM alpine:3.8
WORKDIR /root
COPY --from=builder /go/src/test/main .
CMD ["./main"]- Build
$ docker build -t zev/test:1.0.1 .
Sending build context to Docker daemon 17.41kB
Step 1/9 : FROM golang:1.10.3 as builder
---> d0e7a411e3da
Step 2/9 : WORKDIR /go/src/test
---> Using cache
---> 2b643ce8b3cf
Step 3/9 : RUN go get github.com/gin-gonic/gin
---> Using cache
---> ff970f45de1e
Step 4/9 : COPY src src
---> Using cache
---> 6b79fef06e45
Step 5/9 : RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main src/main.go
---> Using cache
---> 59678a3ab4d8
Step 6/9 : FROM alpine:3.8
---> 11cd0b38bc3c
Step 7/9 : WORKDIR /root
---> Running in 1640c71479d6
Removing intermediate container 1640c71479d6
---> ec68dc839562
Step 8/9 : COPY --from=builder /go/src/test/main .
---> 5bb444c91aff
Step 9/9 : CMD ["./main"]
---> Running in a80305feba6e
Removing intermediate container a80305feba6e
---> 5923597f59c2
Successfully built 5923597f59c2
Successfully tagged zev/test:1.0.1- RUN
$ docker run -it -p 8080:8080 zev/test:1.0.1
[GIN-debug] [WARNING] Now Gin requires Go 1.6 or later and Go 1.7 will be required soon.
[GIN-debug] [WARNING] Creating an Engine instance with the Logger and Recovery middleware already attached.
[GIN-debug] [WARNING] Running in "debug" mode. Switch to "release" mode in production.
- using env: export GIN_MODE=release
- using code: gin.SetMode(gin.ReleaseMode)
[GIN-debug] GET /ping --> main.main.func1 (3 handlers)
[GIN-debug] Listening and serving HTTP on :8080- Images
$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
zev/test 1.0.1 5923597f59c2 2 minutes ago 19.8MB多阶段构建让images缩小了40倍，19.8M的size无论在测试环境还是生产环境都能很好的工作了，但是这样就结束了吗？
当然不是了，我们的目的是让image变得更小，下面看我们的操作。
更小的镜像构建
- Dockerfile:
FROM golang:1.10.3 as builder
RUN apt-get update && apt-get install -y xz-utils \
&& rm -rf /var/lib/apt/lists/*
ADD https://github.com/upx/upx/releases/download/v3.95/upx-3.95-amd64_linux.tar.xz /usr/local
RUN xz -d -c /usr/local/upx-3.95-amd64_linux.tar.xz | tar -xOf - upx-3.95-amd64_linux/upx > /bin/upx && \
chmod a+x /bin/upx
WORKDIR /go/src/test
RUN go get github.com/gin-gonic/gin
COPY src src
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main src/main.go
RUN strip --strip-unneeded main
RUN upx main
FROM alpine:3.8
WORKDIR /root
COPY --from=builder /go/src/test/main .
CMD ["./main"]- Build
$ docker build -t zev/test:1.0.2 .
Sending build context to Docker daemon 17.92kB
Step 1/14 : FROM golang:1.10.3 as builder
---> d0e7a411e3da
Step 2/14 : RUN apt-get update && apt-get install -y xz-utils && rm -rf /var/lib/apt/lists/*
---> Running in 65772cb8fdab
Ign:1 http://deb.debian.org/debian stretch InRelease
Get:2 http://security.debian.org/debian-security stretch/updates InRelease [94.3 kB]
Get:3 http://deb.debian.org/debian stretch-updates InRelease [91.0 kB]
Get:4 http://security.debian.org/debian-security stretch/updates/main amd64 Packages [392 kB]
.....此处省略
Step 10/14 : RUN upx main
---> Running in d802406ee44a
Ultimate Packer for eXecutables
Copyright (C) 1996 - 2018
UPX 3.95 Markus Oberhumer, Laszlo Molnar & John Reiser Aug 26th 2018
File size Ratio Format Name
-------------------- ------ ----------- -----------
9848136 -> 2945384 29.91% linux/amd64 main
Packed 1 file.
Removing intermediate container d802406ee44a
---> 0c29f4b2272d
Step 11/14 : FROM alpine:3.8
---> 11cd0b38bc3c
Step 12/14 : WORKDIR /root
---> Using cache
---> ec68dc839562
Step 13/14 : COPY --from=builder /go/src/test/main .
---> a2c265cc9aff
Step 14/14 : CMD ["./main"]
---> Running in 7e350a4620ee
Removing intermediate container 7e350a4620ee
---> a4d7753c8112
Successfully built a4d7753c8112
Successfully tagged zev/test:1.0.2- RUN
$ docker run -it -p 8080:8080 zev/test:1.0.2
[GIN-debug] [WARNING] Now Gin requires Go 1.6 or later and Go 1.7 will be required soon.
[GIN-debug] [WARNING] Creating an Engine instance with the Logger and Recovery middleware already attached.
[GIN-debug] [WARNING] Running in "debug" mode. Switch to "release" mode in production.
- using env: export GIN_MODE=release
- using code: gin.SetMode(gin.ReleaseMode)
[GIN-debug] GET /ping --> main.main.func1 (3 handlers)
[GIN-debug] Listening and serving HTTP on :8080- Images
$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
zev/test 1.0.2 a4d7753c8112 4 minutes ago 7.36MBOK 非常漂亮，到现在我们已经看到image的size已经缩小到7.36MB,这已经非常的小了，换算过来我们的程序只有2.95M。
我们来一张全景对比看下：
$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
zev/test 1.0.2 a4d7753c8112 6 minutes ago 7.36MB
zev/test 1.0.1 5923597f59c2 About an hour ago 19.8MB
zev/test 1.0.0 a253cfcddd6a About an hour ago 857MB
golang 1.10.3 d0e7a411e3da 6 weeks ago 794MB
alpine 3.8 11cd0b38bc3c 8 weeks ago 4.41MB那么怎么做到的呢，原理很简单，因为alpine的size已经固定了，能让image变得更小的入手点只能是可执行文件，利用UPX的加壳技术可以压缩main可执行程序，可以把main体积缩小50%-70%。
Tip：UPX（the Ultimate Packer for eXecutables）是一个免费且开源的可执行程序文件加壳器，支持许多不同操作系统下的可执行文件格式。想了解更多关于UPX的信息可以点击这里 ，也可点击这里.
总结：
好的，到此为止本文首先展示了单阶段构建镜像，并得到了一个~857MB的镜像，然后利用多阶段构建了一个只包含可执行文件的镜像~19.8MB，最后我们利用多阶段+UPX压缩把我们的镜像缩小到了~7.36MB,这样的镜像无论在测试环境还是生产环境，都一定能大大的提高我们的生产力。
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