You mean I should build a Docker image with both Java and Ruby? The image would be much larger than intended in that case. In production, I would have two different Docker images.
At this time, it is a limitation of the beta that Bitbucket Pipelines can only associate a single image with your build. Avoiding the complexity of chaining images helped us to ship more quickly. I expect this will change before GA. For now, you would have to effectively "merge" the Dockerfiles your self and publish the image so Bitbucket Pipelines can pull it.
It is unfortunately common to ship Docker containers to production that contain the entire tooling suite to build the thing being deployed, and it sounds like that might be what is happening in your case?
If I understand right the thing you're expected to provide to run inside "pipelines" is a container in which your build can be performed; the output of that build might also be a Docker container, but hopefully there is not a requirement that it be the same container in which you are performing the build. Or you might be shipping something completely different, totally unrelated to Docker, as your build output that gets sent onward to production. Of course all of this is just a guess, it will become more clear as some of us start to enter the beta program.
Kyle, this is correct. We use containers simply to create the environment in which we'll execute the script commands in isolation. You can start with a small container and install dependencies during the run or you can prepare a bigger container with the dependencies installed already.