Document how to build and use custom images

[razvan]

Acceptance criteria:

1. Extract the Dockerfile from the lakehouse demo and document it: https://github.com/stackabletech/stackablectl/blob/main/demos/data-lakehouse-iceberg-trino-spark/Dockerfile
2. Document how to use the custom image in Spark applications.

[razvan]

This issue came out during the refinement of stackabletech/nifi-operator#229