Add a "who is this for" documentation section

[Firehed]

While this isn't designed for a niche use-case, there are some specific assumptions that aren't super well-documented at this time:

• Multiple stages (implied by the name, but still...), such as dependency builds
• A stage that produces the server/runtime
• A stage that produces a test environment

Trying to use this without those conditions being met will at best be pointless, so clarifying those expectations in the readme will be valuable.

[gonuke]

Thanks for this action - I think it will be very useful for some of our software stacks. In our case, the notion of a server is not really meaningful as the final output. We are generating docker images with different combinations of dependencies for use in testing our software in CI using Github actions.

I'd be happy to work on vocabulary that might be more generic across different use cases, if you're interested.

[Firehed]

Hi @gonuke - I'm open to feedback/suggestions here!