'docker history' command: Why does the column label say 'IMAGE' when the column contains layers?
It's complicated ;) This post by Nigel Brown is super useful for understanding this, but I'll pull out the relevant points here.
Historically (pre Docker v1.10), each time a new layer was created as a result of a commit action, Docker also created a corresponding image, which was identified by a randomly generated 256-bit UUID, usually referred to as an image ID (presented in the UI as either a short 12-digit hex string, or a long 64-digit hex string).
So, historically they were images, just intermediate ones with no 'human-friendly' tag (although they could be tagged).
Since Docker v1.10, generally, images and layers are no longer synonymous. Instead, an image directly references one or more layers that eventually contribute to a derived container's filesystem.
And if you do a docker history on a pulled image, you'll see something like (taken from the article):
$ docker history swarm
IMAGE CREATED CREATED BY
SIZE COMMENT
c54bba046158 9 days ago /bin/sh -c #(nop) CMD ["--help"] 0 B
<missing> 9 days ago /bin/sh -c #(nop) ENTRYPOINT &{["/swarm"]} 0 B
<missing> 9 days ago /bin/sh -c #(nop) VOLUME [/.swarm] 0 B
<missing> 9 days ago /bin/sh -c #(nop) EXPOSE 2375/tcp 0 B
<missing> 9 days ago /bin/sh -c #(nop) ENV SWARM_HOST=:2375 0 B
<missing> 9 days ago /bin/sh -c #(nop) COPY dir:b76b2255a3b423981a 0 B
<missing> 9 days ago /bin/sh -c #(nop) COPY file:5acf949e76228329d 277.2 kB
<missing> 9 days ago /bin/sh -c #(nop) COPY file:a2157cec2320f541a 19.06 MB
You'll see the IMAGE column reports <missing>, these are not images, but layers that are constituent parts of the image.
So they're not images! What the hell, why is the column named that? (back to your original question). Well, except...:
However, when a layer is committed during an image build on a local Docker host, an 'intermediate' image is created at the same time. Just like all other images, it has a configuration item which is a list of the layer digests that are to be incorporated as part of the image, and its ID or digest contains a hash of the configuration object. Intermediate images aren't tagged with a name, but, they do have a 'Parent' key, which contains the ID of the parent image.
So actually, when you build locally, those constituent layers are images (just like they used to be, even when you pulled them from somewhere else, until v1.10), and are used to facilitate the build cache (part that makes builds fast if you've already built that layer already).
So the answer is...sometimes they are images (technically), sometimes they are layers (and then represented in that column as <missing>). I'd guess it was left as IMAGE for a) historical reasons and b) because they actually are images when something appears there, otherwise it just shows <missing>. I can see how they might be a little confusing, and there certainly might be additional technical detail I've glossed over here, but hope it helps!
Disclaimer: I work for Docker, but my views / posts are my own, etc...