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Sergei Basharov
Asked: 2016-08-25 21:12:16 +0800 CST

What files are pushed to a Docker Hub?

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I am creating and image to reuse in my projects which I want to push to Docker Hub.

I initially thought that I just had to make a folder, put a right Dockerfile with some additional files like configs and push it.

Then I realized that I also had to create an image based on this Dockerfile first.

So, I suppose, Docker Hub keeps the image that I created locally? Thus, images can be quite large, like gigabytes?

Please shed light on what's exactly is being pushed when I run docker push name/repo.

docker

1 Answers

  1. A good start to understanding is checking out Docker's overview of images and containers.

    In summary - Docker is built on a layered filesystem. Each command in a Dockerfile adds, writes or modifies files, which are stored as changes to the layer below. This process makes it easy to re-use base containers (because the layers are not modified) - and efficiently rebuild images when commands have not changed.

    And those filesystem layers (plus other metadata that make up an image) are what is pushed to Docker Hub.

    Thus, images can be quite large, like gigabytes?

    A small Dockerfile often creates a much larger image, so why not upload just the Dockerfile? I think the reason is consistency. Often a Dockerfile starts with FROM debian or FROM alpine or maybe with a version tag like FROM python:3.5. But when the image behind that tag changes, you will build different images from the same Dockerfile. At some point a build could break an application, or cause subtle bugs. But with all the filesystem layers cacged on Docker Hub, the same binary image runs every time.

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