Add Puma via govuk_app_config

This commit adds `Puma` via govuk_app_config. Puma was previously installed indirectly through the dependency on `govuk_test` as part of the `dev` and `test` groups; given that we want to run Puma in production, we are instead configuring it via govuk_app_config.

Co-authored-by: Chris Banks <chris.banks@digital.cabinet-office.gov.uk>

Dockerfile

@@ -4,7 +4,6 @@ RUN apt-get update -qq && apt-get upgrade -y
 RUN curl -sL https://deb.nodesource.com/setup_9.x | bash -
 RUN apt-get update && apt-get install -y nodejs && apt-get install npm -y
 RUN npm install -g phantomjs-prebuilt@2 --unsafe-perm
-RUN gem install foreman
 
 # This image is only intended to be able to run this app in a production RAILS_ENV
 ENV RAILS_ENV production
@@ -28,4 +27,4 @@ RUN GOVUK_APP_DOMAIN=www.gov.uk GOVUK_WEBSITE_ROOT=www.gov.uk bundle exec rails
 
 HEALTHCHECK CMD curl --silent --fail localhost:$PORT || exit 1
 
-CMD foreman run web
+CMD bundle exec puma

config/puma.rb

@@ -0,0 +1,2 @@
+require "govuk_app_config/govuk_puma"
+GovukPuma.configure(self)