Support for custom user IDs in Envbox workspaces

It would be helpful to have the ability to customize the user ID (UID) for workspaces. This would help customers that need a way to assign specific UIDs to users when starting workspaces, in order to match permissions on their mounted drives.

**Current state:** Envbox automatically detects and uses the "coder" user from the image

**Ideal State:** customers are able to create a custom user with a specific UID during the workspace startup process. This would allow them to properly set permissions for accessing NFS-mounted directories with existing Linux-style UID-based permissions.