But are containers portable? Can you take them, send them per mail or put on a USB-stick? And I'm not talking about the recipe for creating them, but the actual container itself.

Yes.

The actual image is a tar file, and container runtimes include commands for both exporting and importing. Exceptionally portable. More portable than JAR files and Go binaries, at least.

Wait, which container are we talking about? Docker? AppImage? Kubernetes? What about the runtime?

OCI Images. You can run them in Docker, Podman, Kubernetes, systemd-nspawn, etc.

Nobody is going to use your tool if they have to fetch it from a container registry and have to run it isolated from their filesystem and network, though. OCI images are fine if you have a pet app and you have to ship it to a production server. Not that convenient for interactive tools, though. Consider:

  $ docker run -v /tmp/foobar:/tmp/foobar debian:stable ls /tmp/foobar
  a.txt

You really just want to type "ls /tmp/foobar".

That's something that aliases excel at.