These modifications will allow you to restart a container that is using puma as the app server. Create a `script` directory (`mkdir script`), and add the `start` script to it. `start` will need to be made executable (`chmod +x script/start`). The only line in the Dockerfile that should change is the `CMD` line. Rebuild the `app` image.

Dockerfile

FROM ruby:2.3.1

RUN apt-get update -yqq \
  && apt-get install -yqq --no-install-recommends \
    postgresql-client \
    nodejs \
  && apt-get -q clean \
  && rm -rf /var/lib/apt/lists

# Pre-install gems with native extensions
RUN gem install nokogiri -v "1.6.8.1"

WORKDIR /usr/src/app
COPY Gemfile* ./
RUN bundle install
COPY . .

CMD script/start

start

#!/bin/bash -e

if [[ -a /tmp/puma.pid ]]; then
  rm /tmp/puma.pid
fi

rails server -b 0.0.0.0 -P /tmp/puma.pid