docker_build ref injection doesn't inject into :latest when two images

Reported by Cameron in the tilt channel:

He had a Tiltfile with two docker_builds:

docker_build(image, ...)
docker_build(image + ':init', ...)

He also had Kuberentes YAML that referenced "image:latest" and "image:init".

Tilt complained that the image was not used in any deploy config. This was confusing, because this worked fine with the plain "docker_build(image)"

Workaround

The workaround is to change docker_build(image) to docker_build(image + ":latest")

Possible fixes

There's two different fixes here. We could do one or both.

1. Make docker_build(image) match image:latest when there are multiple image builds for the same name. This seems like a reasonable expectation (given that :latest is implied in other contexts)
2. Improve the error message for the case when you the image name matches but the tag name does not match

Start by tracing the docker_build image/tag matching and ref-injection path, using the two-image example with image:latest and image:init as the reproduction. Done means docker_build(image) correctly matches image:latest alongside another tag, or the error clearly explains the tag mismatch; the issue names no files or tests to run.