How to add custom transformer to mleap-serving docker

Need some clarity,

This is what I did:
- Define custom transformer classes (Core, spark, sparkOp, mleap-runtime, mleapOp)
- Added both spark and mleap to `reference.conf`
- sbt package (creates the jar for me)
- Was able to run the transformer in spark (adding the jar created by sbt to spark)

After this I tried to load bundle in the spring-boot docker container and realized that the mleap-spring-boot app does not have my custom transformer class in its runtime.

Questions:
- How does the registry work? Is it a registry for bundle/for transformer code or what?
- How do I get my classes into the mleap-serving / mleap-spring-boot runtime (in docker image)?

I poked around the mleap codebase and docker container and found `/opt/docker/lib` directory where all the jars are stored. Should I copy my custom transformer jar to this directory when I build my custom docker image (built from the mleap base docker image)?

Start with the custom entries in reference.conf, the mleap-spring-boot runtime, and the /opt/docker/lib directory described in the issue. Compare the jar produced by sbt package with the mleap base Docker image and determine how the registry and runtime classpath are handled. Done means the supported process for packaging and loading a custom transformer is documented.

Written by the indexing model from the issue text.