This action targets a very specific use-case that is not currently supported by Github Workflows. This action gives you the capability to run built containers.
Docker already supports running commands inside a docker image. See [jobs.<jobs_id>.container](https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idcontainer). But it doesn't give you a clean way to run an image from a private repo or an image built on a previous step.