Need help connecting docker and puppeteer for unit testing

I am busy with a project. The goal of my project is automated testing using puppeteer and docker. I have trouble with implementing the correct info in my dockerfile and in my code. When I run my container I see that it runs and I dont get error messages but I don’t see the program output. Hope that someone can show me the direction where i need to look at. Every help is appreciated even some links to good tutorials.

Dockerfile:

FROM kinlan/puppets:latest
# Create app directory
WORKDIR /

# Install app dependencies
# A wildcard is used to ensure both package.json AND package-lock.json are copied
# where available ([email protected]+)
COPY package*.json ./

RUN npm install
# If you are building your code for production
# RUN npm ci --only=production

# Add user so we don't need --no-sandbox.
RUN groupadd -r pptruser && useradd -r -g pptruser -G audio,video pptruser 
    && mkdir -p /home/pptruser/Downloads 
    && chown -R pptruser:pptruser /home/pptruser 
    && chown -R pptruser:pptruser ./node_modules


# Run everything after as non-privileged user.
USER pptruser

# ENV CHROME_BIN=/usr/bin/google-chrome

# Bundle app source
EXPOSE 9876
CMD [ "npm", "test" ]

Source: StackOverflow