chore: add dockerignore for leaner builds (#1236)

[mittalsonal]

Description\n- add a root .dockerignore for Docker build contexts\n- exclude local dependencies, build output, env files, caches, and repo-only metadata from Docker sends\n- keep the change scoped to Docker build hygiene only\n\n## Related Issue\n- Closes #1236\n\n## Checklist\n- [x] one-file change\n- [x] git diff --check passes\n- [x] excludes only non-essential local/dev files from Docker context\n\n## Pillar\n- Open Source\n

[vercel]

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[mittalsonal]

Kind request for GSSoC labels when reviewed: gssoc:approved, one level:* label, and ype:devops if it fits the repo’s scoring rubric. Thanks!

[mittalsonal]

Verification completed locally:

• git diff --check

This PR intentionally keeps the change small and limited to a root .dockerignore for cleaner Docker build contexts.