Add `healthcheck` support

[riken127]

Title

Add healthcheck support to DSL and emitters

Description

Healthchecks are essential for production-grade containers.
Support should include:

• DSL syntax
• Dockerfile generation
• Compose generation

Acceptance Criteria

• AST support for healthcheck
• Semantic validation
• Dockerfile emitter support
• Compose emitter support
• Tests covering typical cases