Add --data-root flag for custom volume bind mount paths

[conorbranagan]

Description

Add support for a --data-root flag to supabase start that enables custom host paths for persistent data instead of using Docker named volumes. This allows Supabase to run in container-in-container (DinD) environments where Docker daemon configuration cannot be easily modified.

Changes

Features

• Add --data-root CLI flag to supabase start command
• Support data_root configuration option in config.toml under [local] section
• Convert named volumes to bind mounts when --data-root is specified:
  • Database: <data-root>/db:/var/lib/postgresql/data (replaces supabase_db_[ProjectId] named volume)
  • Storage: <data-root>/storage:/mnt (replaces supabase_storage_[ProjectId] named volume)
• Automatically create required subdirectories if they don't exist
• CLI flag takes precedence over config file setting

Files Modified

• cmd/start.go - Added --data-root flag definition
• internal/start/start.go - Updated Run() to accept and apply dataRoot parameter, added createDataDirectories() helper function
• internal/db/start/start.go - Modified NewHostConfig() to use bind mounts when DataRoot is configured
• pkg/config/config.go - Added DataRoot field to Local config struct
• pkg/config/templates/config.toml - Added data_root configuration example
• internal/db/start/start_test.go - Added tests for host config with and without DataRoot
• internal/start/start_test.go - Added test coverage for data directory creation

Testing

Added unit tests covering:

• Default behavior using named volumes when DataRoot is not set
• Bind mount behavior when DataRoot is configured
• Data directory creation

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