chatbot-rag-app: publish docker image

[codefromthecrypt]

The main chatbot-rag-app example has instructions to build a docker image, but that should only be done during dev. We need to publish an image so that the instructions are easier, particularly for #366

@anuraaga can you work through this tomorrow?

• Get access from @estolfo or @joemcelroy to publish and mark image public on ghcr.io for this repo - @codefromthecrypt has it now
• Create a PR that publishes a docker image on non-README (docs) changes in the chatbot-rag-app directory. Add workflow to build chatbot-rag-app docker image #394
• Update the chatbot-rag-app README to use the published image, moving --build hints to the bottom (heading like "running in docker from source"). Use published images in chatbot-rag-app example #395
• Update the blog and samples to use this image on https://github.com/elastic/observability-labs/pull/332 cc @bshetti
  • Update chatbot-rag-app example to use published image observability-examples#49
  • https://github.com/elastic/observability-labs/pull/345

[codefromthecrypt]

thanks for all the work @anuraaga!