Add container jobs and convert action to 'composite'

[eine]

This is a significant rewrite of this repo:

• Two additional jobs are added to the workflow, for showcasing how to run a job in a container, and how to run a single step of a job in a container.
• The Action is converted from a 'Docker action' to a 'Composite run steps Action' (https://docs.github.com/en/free-pro-team@latest/actions/creating-actions/creating-a-composite-run-steps-action). That allows users to provide the image to be used, which is a new optional input.

The README is updated according to the enhancements.