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Why does docker create containers while building images?

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Why does docker start a container for every command in Dockerfile? I understand a new layer is created for every command.

Step 1/3 : FROM nginx:latest
latest: Pulling from library/nginx
8ec398bc0356: Pull complete 
dfb2a46f8c2c: Pull complete 
b65031b6a2a5: Pull complete 
Digest: sha256:8aa7f6a9585d908a63e5e418dc5d14ae7467d2e36e1ab4f0d8f9d059a3d071ce
Status: Downloaded newer image for nginx:latest
 ---> c7460dfcab50
Step 2/3 : WORKDIR /usr/share/nginx/html
 ---> Running in f8adb5dc5a47
Removing intermediate container f8adb5dc5a47
 ---> a59fb35a43c2
Step 3/3 : COPY . .
 ---> 7911d9a451d9
Successfully built 7911d9a451d9
Successfully tagged mynginx:latest

What is the point of creating a new container and setting WORKDIR in it when you are gonna remove that container anyway?

Notice this line Removing intermediate container f8adb5dc5a47

like image 788
kooskoos Avatar asked Jan 17 '20 12:01

kooskoos

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1 Answers

For each line daemon creates a new image and each instruction runs independently. Docker Daemon uses intermediate images to accelerate the docker build process. Build cache indicates this.

like image 188
Sanket Singh Avatar answered Sep 30 '22 19:09

Sanket Singh
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