docs(self-hosting): add GSSoC docker variables reference

[Honey-pg]

Summary

Adds a standardized Docker variables reference/manual section to docs/self-hosting.md for GSSoC contributors and self-hosting users.

Changes

• Added a dedicated Docker variables reference table
• Documented required Docker/self-hosting environment variables
• Clarified NODE_ENV=production for container deployments
• Kept the guide aligned with the existing docs structure

Notes

• Documentation-only change
• No code behavior changed

Closes #1957

[vercel]

@Honey-pg is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[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

[github-actions]

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