Dependency image not rebuild when base image changes.

[codeviking]

The dependency image doesn't rebuild when the base image changes.

Our use case:

1. I published a new version of the allenai-docker-private-docker.bintray.io/oracle-java:8 image.
2. I updated our "base" image by pulling the latest of that image and rebuilding (and republishing our image).
3. I ran dockerRun, but the latest base image wasn't pulled. I had to instead run clean and then dockerRun.