How to combine two containers?

I have a Golang on Ubuntu server application that runs ok.
I want to move it to docker. This version however requires Mongo to work.
I am having a hard time getting both to work together.
Do i need two separate containers?
Can i put the Mongo inside the Ubuntu server with Golang server?

How would i combine these two Dockerfiles?

# Set the base image to Ubuntu
FROM ubuntu

WORKDIR /go
COPY . .

# Expose the default port
EXPOSE 8000
EXPOSE 80

CMD ["./awszipperv2go"] 

and

#####  work on mongo
FROM mongo
COPY . /mongo-seed

ENTRYPOINT mongod