docs: expand Docker Compose section and fix YAML comments

[bartlomieju]

Summary

• Adds a multi-service Docker Compose example (Deno app + Postgres) with health checks, volumes, and depends_on
• Removes deprecated version key from Compose examples
• Fixes invalid // filepath: comments in YAML/text blocks (YAML uses #)

Closes #2863

Test plan

• Verify Compose examples render correctly
• Verify other code blocks with fixed comments render correctly

🤖 Generated with Claude Code

[lunadogbot]

Replacing // filepath: (invalid YAML) with title="docker-compose.yml" is the right fix, and the second Compose example with the long-form depends_on.db.condition: service_healthy plus pg_isready healthcheck is valid against Compose v2.

• nit: the .dockerignore, build-context.sh, and Dockerfile blocks switched to # filepath:… comments inside the code rather than the title="…" attribute used by the two yaml blocks above. title= works on non-yaml blocks too (see e.g. text title="log.txt" elsewhere in the repo), so moving these three to title="docker/project-a/.dockerignore" etc. would match the yaml style instead of mixing two conventions in one page.
• nit: last_modified: 2025-12-16 in the frontmatter wasn't bumped — adding the ~39-line Postgres Compose example is substantive per the repo's pattern.