Add a base image for integration test suite for consumption in CircleCI

[ebramanti]

We should add a base image that we can use in CircleCI for integration so we don't have to download dependencies on every run. The image in integration/Dockerfile could be broken apart into a separate base image, and then consumed by that Dockerfile and the Circle build step.

The only downside of this change is that we would have to manually publish for testing a branch that adds a dependency to Ship. An alternative to this could be building the Docker image on every CI run and manually caching the layers (see here).