Merging Docker Images

docker compose?

Unless you need inprocess communication (you don't for NodeJS native API) you can just refer from one container to another.

There is no such way as merging two images. If you need only one image, the only way is to make a new one, which would combine the main from both images. So, you should choose which one is going to be as a base image, I would suggest it supposed to be IRIS. Then you can create install inside nodejs, and your node application. The issue is here you may face is that you will have only one init process after that. While I suppose you have to have started IRIS and NodeJS application at the moment. You can add a script which will run in a background nodejs application, and pass it to iris-main through CMD directive in conjunction with '-a', '-b', '-c' or '-t' parameter
So, it can be something like this

FROM intersystestems/iris:2020.2.0.204.0

USER root 

# script which will run nodejs application in background
COPY app-entrypoint.sh /

# copy your nodejs application
COPY ./nodeapp /opt/nodeapp

# install nodejs v10, and node_modules
RUN apt-get -y update && \
 DEBIAN_FRONTEND=noninteractive apt-get -y install curl --no-install-recommends && \
 curl -sL https://deb.nodesource.com/setup_10.x | bash - && \
 DEBIAN_FRONTEND=noninteractive apt-get install -y nodejs --no-install-recommends && \
 rm -rf /var/lib/apt/lists/* && \
 chmod +x /app-entrypoint && \
 cd /opt/nodeapp && \
 npm install

USER ${ISC_PACKAGE_MGRUSER}

# say to start your nodejs application after IRIS start
CMD ["-a", "/node-entrypoint.sh"]