Document Docker build args usage (VERSION, BUILD_TIME)

[adnaan]

Context

This follow-up task was identified during the review of PR #132.

Source PR: #132
PR Title: feat: add Docker support with multi-stage build
Suggested by: @claude[bot]

Task Description

The Dockerfile defines VERSION and BUILD_TIME build arguments but they are not documented.

Add documentation showing users how to pass these during build:

docker build \
  --build-arg VERSION=$(git describe --tags) \
  --build-arg BUILD_TIME=$(date -u +%Y-%m-%dT%H:%M:%SZ) \
  -t tinkerdown .

Acceptance Criteria

• README or docs include build arg examples
• Port mapping documentation included

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This issue was automatically created from PR review comments.