Chain Docker images

Each subdirectory represents a Docker build context. Any file in
a build context is available to commands in its corresponding
Dockerfile. This allows the Dockerfile to copy these files into
the image. All files within a context will be used inside its
resultant container.

`startup.sh` is used as the ENTRYPOINT to its corresponding Dockerfile.
This shell script will be the first command run on container startup.

Each container may have an accompanying shell script in `chain/bin`.
This script builds other assets (for example, the cored binary) and
copies them into the build context before building the image.