I have a docker image with this command:
FROM ruby:2.4-alpine
WORKDIR /usr/src/app
COPY Gemfile /usr/src/app/Gemfile
COPY Gemfile.lock /usr/src/app/Gemfile.
/usr/src/app may not be in your path so you should include the full path to the script. You also need to ensure that your entrypoint.sh is executable, docker will copy the permissions exactly as they are on your build host, so this step may not be needed depending on your scenario.
FROM ruby:2.4-alpine
WORKDIR /usr/src/app
COPY Gemfile /usr/src/app/Gemfile
COPY Gemfile.lock /usr/src/app/Gemfile.lock
RUN bundle config build.nokogiri --use-system-libraries
RUN bundle install --without development test
VOLUME /state
COPY . /usr/src/app/
RUN chmod 755 api-entrypoint.sh
ENTRYPOINT ["/usr/src/app/api-entrypoint.sh"]
CMD ["foreman", "start"]