build this as docker image build -t render:dev .
then use this as a base image, placing root content at /app/content/README.md:
Dockerfile:
FROM render:dev
ARG SOURCE
COPY $SOURCE /app/content
CMD npm start
Build: docker image build -t myimage:xxx --build-arg SOURCE=mysource .
Launch: docker container run --rm -d -p 8000:8080 myimage:xxx