How to use Dockerfile to link main container to a db container?

Docker has a quite convenient way to link the main container to a db container with a command like:

docker run --link db:db user/main

This is very convenient already. However, I believe it’s still clumsy compared to a command like:

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  • docker run user/ultra
    

    where ultra is a container that is already linking the main container to the db container.

    Is that possible that I can achieve this by writing a good Dockerfile.

    I suppose I can start the Dockerfile with

    FROM user/main 
    

    but how do I get the second container involved and then link them with Dockerfile?

    Thanks.

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