Dockerfile : FROM scratch

就是一個空的，什麼都沒有的 base image。
所以如果要用來 run 自己的 program，一定要是 static linked。
最好的說明就是docker 的hello-world image。
clone 下來以後，到 amd64/hello-world 看，就是做出 hello-world image 的 程式(hello) 和 Dockerfile。
用 file 來看 hello ，就是 statically linked, executable program

Dockerfile 就很簡單：

FROM scratch
COPY hello /
CMD ["/hello"]

然後 build image:

docker build -t myhello . --no-cache

run image:

docker run myhello

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
    (amd64)
 3. The Docker daemon created a new container from that image which runs the
    executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which sent it
    to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

這個 image 很小，就跟 hello 一樣大：

docker images
EPOSITORY                              TAG                            IMAGE ID       CREATED          SIZE
myhello                                 latest                         03cc4ebe3142   48 seconds ago   13.3kB

ls -l hello 
-rwxrwxr-x 1 charles-chang charles-chang 13256 Sep 27 15:55 hello

---

另一個，簡單，用 go 做 example。一樣的 hello :building minimal docker image for go application

main.go:

package main

import (
	"fmt"
	"time"
)

func main() {
	for {
		fmt.Println("Hello World!")
		time.Sleep(1 * time.Second)
	}
}

build for static:

$CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -o main

然後Dockerfile 一樣：

FROM scratch
ADD main /
CMD ["/main"]

然後就跟上面一樣，build image, run ....

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以前好像寫過，go cross build 好方便，上面那個 GOARCH=arm64 就是 build for aarch64