What are Docker image “layers”?

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I might be late, but here’s my 10 cents (complementing ashishjain’s answer):

Basically, a layer, or image layer is a change on an image, or an intermediate image. Every command you specify (FROM, RUN, COPY, etc.) in your Dockerfile causes the previous image to change, thus creating a new layer. You can think of it as staging changes when you’re using git: You add a file’s change, then another one, then another one…

Consider the following Dockerfile:

FROM rails:onbuild
ENV RAILS_ENV production
ENTRYPOINT ["bundle", "exec", "puma"]

First, we choose a starting image: rails:onbuild, which in turn has many layers.
We add another layer on top of our starting image, setting the environment variable RAILS_ENV with the ENV command. Then, we tell docker to run bundle exec puma (which boots up the rails server). That’s another layer.

The concept of layers comes in handy at the time of building images. Because layers are intermediate images, if you make a change to your Dockerfile, docker will rebuild only the layer that was changed and the ones after that. This is called layer caching.

You can read more about it here.

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