Update action to be JS-based

[MelSumner]

Summary

Update action to be JS based, so Docker is no longer needed.

Action Items

• refactor Dockerfile into JS
• update docs that reference Docker (README, CONTRIBUTING)
• update action.yml
• update github workflow build.yml
• deprecate Docker (Remove docker repo & images)

[benschwarz]

As a prerequisite, we'll need to remove mozjpeg, then we can action this change.

We might need to double check all the action inputs are handled correctly in the transition from Docker to plain ol' JS, but… I think it'll be ok.

[MelSumner]

@benschwarz I tried to handle that prereq with some PRs- #285 and #288

I thought once we had something that at least built correctly, we could then work to refactor and update. WDYT?

[benschwarz]

@MelSumner absolutely! If that works for you, let's go that way 👍