This is something I’ve needed to get off my chest; I really and truly don’t understand the logic behind the scheme of Docker container image names. The first time I tried to grasp it, I got mentally stuck, because it feels to me it blatant ignores any convention. At the moment it is a matter of accepting that this it how it works, but my neurotic side keeps protesting every time I tag an image. Let me explain the beef I have, and then maybe someone can enlighten me.

(N.B. I’m using “docker” in this post, but I actually am using “podman”. But their APIs are identical, so because docker is better known, I’m typing “docker” in this post.)

When you create a new image using the “docker build” command, the newly created image is uniquely identified by a hash, something in the line of 21df6af2a82e0135661ce18a621112450992914e1fcfc0edb285c332318e3e23. This id is totally impossible to convey to other people, so Docker has the possibility to add tags to images. A typical build command looks like

$ docker build --tag my-tag .

This is will use the Dockerfile (Containerfile for podman) to generate an image and add it to the local registry. You can see this by executing a list command:

$ docker images

REPOSITORY        TAG             IMAGE ID      CREATED         SIZE
localhost/my-tag  latest          99bd5445f871  22 minutes ago  280 MB

Wait! What? What is the localhost doing there? And latest?