The problem is that getporter/porter-hello-installer is the invocation image, and explain requires that you pass in the tag for a bundle. Try the command again and pass in this instead getporter/porter-hello:v0.1.0 and you will see output

$ porter explain -t getporter/porter-hello:v0.1.0
Name: HELLO
Description: An example Porter configuration
Version: 0.1.0

No credentials defined

Parameters:
Name           Description                                                     Type      Default   Required   Applies To
porter-debug   Print debug information from Porter when executing the bundle   boolean   false     false      All Actions

No outputs defined

No custom actions defined

It would be great if porter detected the type of the image pulled, and if it's a regular docker image, it should give you a better error message like "--tag should be the tag of a bundle, but an image tag was specified".

I see error coming directly from dependency package: cnab-to-oci . I don't think porter can detect what type of image it is

cnab-to-oci is a dependency of ours that we can contribute to in the cnabio org, similar to cnab-go. So no worries, we aren't limited to just solutions that are within Porter's codebase.