Each line in a Dockerfile is a layer in an image

What has helped me grok docker a bit better is knowing that every line in a Dockerfile has a corresponding hash identifier after the image has been built. Here is a sample Dockerfile:

FROM alpine

RUN echo 'helloworld' > helloworld.txt

CMD ["cat", "helloworld.txt"]

I create the image with:

docker build -t helloworld .

I can now examine each layer in the Dockerfile with docker history helloworld

IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
1e5d27ca20a8        13 hours ago        /bin/sh -c #(nop)  CMD ["env"]                  0B
84f489011989        13 hours ago        /bin/sh -c echo "Hello World" > helloworld.t…   12B
3fd9065eaf02        2 months ago        /bin/sh -c #(nop)  CMD ["/bin/sh"]              0B
<missing>           2 months ago        /bin/sh -c #(nop) ADD file:093f0723fa46f6cdb…   4.15MB

Three commands and four layers. The FROM alpine command is actually 2 layers that have been squashed together. You can see the <missing> hash for the initial command because it has been squashed into 3fd9065.

The command that creates the helloworld.txt file has a size of 12 bytes because thats the size of the file that was created.

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