Does Docker EXPOSE make a new layer?

Question

I have been playing around with creating docker files and reading the documentation and I was wondering this question: Does adding an EXPOSE command to my Dockerfile add a layer? (and if it does why would I care/ does it matter where it is placed inside the file?)

It is not specifly said in the documentation.

I understand RUN, COPY and ADD create layers because they change the file system, but expose simply adds metadata to the container, does it's change generates a layer?

Answer 1

Yes, every instruction in a Dockerfile generates a new layer for the resulting image.

However, layers created via EXPOSE are empty layers. That is, their size is 0 bytes.

While they don't impact you storage-wise, they do count for leveraging layer cache while building or pulling/pushing images from a registry.

A good way to understand an image's layers is to use the docker history command. For instance, given the following Dockerfile:

FROM scratch

EXPOSE 4000
EXPOSE 3000

do

docker build -t test/image .

If you then docker history test/image you'll see:

IMAGE               CREATED             CREATED BY                           SIZE                COMMENT
ab9f435de7bc        4 seconds ago       /bin/sh -c #(nop)  EXPOSE 4000/tcp   0 B                 
15e09691c313        5 seconds ago       /bin/sh -c #(nop)  EXPOSE 3000/tcp   0 B     

If you switch the order of the EXPOSE statements and build again, you'll see the layer cache being ignored.