Installing mongo client in a Docker container

I am using fig to build and run my app within various Docker containers and so-far, so good.

I have a container for my app and a db container with mongo in it.

But now I am trying to connect to the mongo server to seed it with a user and database and I can't find how to install the mongo client.

My app's image is derived from the standard dockerfile/nodejs image and that does not include a mongo client.

Trying RUN mongo just gives an error mongo not found.

All the documentation I've turned up so far relates to how to install the mongo server within a Docker container, and nothing I;ve found so far tells me how to install the mongo client. How do I do this?

What about:

RUN sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
RUN sudo apt-get install -y mongodb-org-shell
RUN sudo apt-get install -y mongodb-org-tools

as suggested here?