In the docs folder...

.dockerignore

.git
source

docker-compose.yaml

app:
  build: .
  ports:
    - 4567:4567
  volumes:
    - .:/usr/src/app

Dockerfile

FROM ruby:2.3-alpine
COPY . /usr/src/app
VOLUME /usr/src/app
EXPOSE 4567

WORKDIR /usr/src/app

RUN apk add --update nodejs g++ make
RUN bundle install

CMD ["bundle", "exec", "middleman", "server", "--watcher-force-polling"]

We should use FROM ruby:2.5-alpine in Dockerfile.
This might be helpful too. https://medium.com/technical-writing-is-easy/how-to-build-an-api-docs-site-using-slate-2e1428780076