test updated build-push-ecr action

[boringcactus]

Summary

Ticket: none

I'm working on updating mbta/actions/build-push-ecr to be based on docker/build-push-action and adding caching via GitHub Actions. Since the docker-additional-args input allows for arbitrary command line flags, but docker/build-push-action doesn't expose such a mechanism, maintaining backwards compatibility is something of an adventure. Conveniently, this repo and Skate are the only places that pass docker-additional-args, and both of them only use --build-arg, so I've done some creative shell scripting to translate into the more straightforward build args docker/build-push-action expects. There'll be an actual migration process eventually, but in the short term, I need to be confident my creative shell scripting actually works, which means running my branch of mbta/actions/build-push-ecr to deploy this repo to some dev instance so we can validate that the build args are going in as specified.

This PR should not be merged, just temporarily deployed to a dev environment long enough to validate that the build args and additional tags get applied correctly. After mbta/actions#41 is merged, I'll come back and make a separate PR to actually migrate to the new build args format so I can get rid of the creative shell scripting.

Testing

I've checked the new action's shell scripting setup locally and confirmed that it should in theory work.

[anthonyshull]

@boringcactus can we delete this PR?

[boringcactus]

Sure.