feat(create-app): add --starter flag to pass custom starter URL to local Docker instance

Description

Add a --starter <url> CLI flag to dotcms-create-app so users can specify a custom starter ZIP when spinning up a local dotCMS Docker instance. The value maps directly to the CUSTOM_STARTER_URL environment variable already supported in the docker-compose file.

Acceptance Criteria

• --starter <url> option is available in dotcms-create-app
• When provided, CUSTOM_STARTER_URL is set in the Docker environment before running docker compose up
• The starter URL is validated (must be a valid http/https URL)
• Omitting --starter preserves existing behavior (uses the default starter from docker-compose)

Additional Context

PR: #35025
Relevant docker-compose env var: CUSTOM_STARTER_URL