feat: containerize the Katra application

[ibourgeois]

Context

As a maintainer, I need Katra to have a supported container build target so the project can publish a deployable Docker image and use container-native CI workflows.

Acceptance Criteria

• The repository includes a supported Docker build for the application.
• The containerization approach defines required runtime services, entrypoints, and environment expectations.
• Local and CI build flows are documented well enough to support image publishing.
• The chosen image shape aligns with the project's Laravel and deployment direction.

Notes

This issue is the application packaging prerequisite for publishing images to GitHub Container Registry.

Out of Scope

• Kubernetes manifests
• Production infrastructure provisioning
• Multi-service orchestration beyond what is needed to define the image contract