Docs: needs more clarifications on default behaviour

[ahmadnassri]

The README currently describes the the two different usage of Git context and Path context..

however it doesn't clearly / explicitly explain that the default behavior is to checkout the repository at the current github ref (based on the event that triggered the action) and use that to build the image.

meaing that any file mutation / creation / updates to the repo in steps that preceed the build step are completely lost and ignored by the build action despite using the github/checkout action in previous steps.

I'd argue this is not a clear and obvious default for the average user who at first glance by looking at the README file, AND given the reset of the GitHub Actions ecosystem defaults ...

if changing the default behaviour is too big of a change, then at least consider updating the docs to explicitly state this and indicate the need to set the context and file arguments as well as the github/checkout step.

I would even include examples in the README of content modification in the repo before the build step.

thanks