'Unhandled promise rejection' running a dockerised node app

Moving on from this issue, I removed the images and ran docker build. The build completed successfully and so I ran docker run -it --rm -p 8080:8080 cd3:0.0.3

The run seems to go OK and reaches the point of the app listening, but then it errors and the container fails.

to be executed: node ./index.js
==> 🌎  Listening at http://localhost:8080
(node:158) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: Exited with code 3
(node:158) DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
^Cnpm info lifecycle clothes_doctor@0.0.3~poststart:production: clothes_doctor@0.0.3
npm info ok

Does anyone know what's happening here?

Dockerfile:

FROM node:7.1.0

RUN mkdir -p /code
WORKDIR /code
COPY . /code
RUN npm install -g -s --no-progress yarn && \
    yarn && \
    npm rebuild node-sass --force
CMD [ "npm", "run", "start:production" ]
EXPOSE 8080

via user1775718