Podman Fully Qualified Image Names

I installed Podman.  Then, of course, I tried to pull an image, but not the example ubuntu image.  This immediately generated an error:

Error: short-name "golang" did not resolve to an alias and no unqualified-search registries are defined in "/etc/containers/registries.conf"

(The “ubuntu” example the tutorial used is actually configured as a short-name in the file /etc/containers/registries.conf.d/shortnames.conf on my system. It’s also possible that later Podman versions will prompt for a repository.)

Luckily, there’s a fairly straightforward way to convert the image names to fully-qualified ones that Podman will accept.

Two basic concepts apply:

1. Docker images are always on docker.io
2. Docker images without a prefix (as in “golang” or “postgres”) are under library

That is, once we rewrite Docker commands for Podman, they might look like:

podman pull docker.io/library/postgres:alpine
podman pull docker.io/gitea/gitea:latest

These are the equivalent to pulling postgres and gitea/gitea with Docker.

If you’re looking at an image on Docker Hub, and its URL is something like hub.docker.com/_/golang, then the _ is the “lack of prefix.”  It indicates that the URL for Podman will be docker.io/library/golang.

If the URL is, instead, hub.docker.com/r/gitea/lgtm, then that shows the prefix (user gitea) and container (lgtm) after the /r/: the final URL for Podman will be docker.io/gitea/lgtm.

The final thing to note is that this information applies to the FROM directive in a Dockerfile as well, and it’s compatible with Docker.  Docker and Podman both happily accept FROM docker.io/library/golang:1.19-alpine.