Use bootsnap to reduce resource usage spike on startup.

Bootsnap has been enabled as standard for new Rails apps since Rails
5.2. It reduces startup time and - more importantly - reduces spikes in
resource usage at startup
([example](https://www.github.com/alphagov/government-frontend/pull/2626)).

Dockerfile

@@ -9,6 +9,7 @@ WORKDIR $APP_HOME
 COPY Gemfile* .ruby-version ./
 RUN bundle install
 COPY . .
+RUN bootsnap precompile --gemfile .
 
 
 FROM $base_image
@@ -17,6 +18,7 @@ ENV GOVUK_APP_NAME=publishing-api
 
 WORKDIR $APP_HOME
 COPY --from=builder $BUNDLE_PATH $BUNDLE_PATH
+COPY --from=builder $BOOTSNAP_CACHE_DIR $BOOTSNAP_CACHE_DIR
 COPY --from=builder $APP_HOME .
 
 USER app

Gemfile

@@ -3,6 +3,7 @@ source "https://rubygems.org"
 gem "rails", "7.0.4"
 
 gem "aws-sdk-s3"
+gem "bootsnap", require: false
 gem "bunny"
 gem "dalli"
 gem "fuzzy_match"

Gemfile.lock

@@ -88,6 +88,8 @@ GEM
     aws-sigv4 (1.5.2)
       aws-eventstream (~> 1, >= 1.0.2)
     bindex (0.8.1)
+    bootsnap (1.15.0)
+      msgpack (~> 1.2)
     brakeman (5.2.3)
     brpoplpush-redis_script (0.1.3)
       concurrent-ruby (~> 1.0, >= 1.0.5)
@@ -247,6 +249,7 @@ GEM
     mini_mime (1.1.2)
     mini_portile2 (2.8.1)
     minitest (5.17.0)
+    msgpack (1.6.0)
     multi_xml (0.6.0)
     net-imap (0.3.4)
       date
@@ -532,6 +535,7 @@ PLATFORMS
 
 DEPENDENCIES
   aws-sdk-s3
+  bootsnap
   bunny
   byebug
   climate_control

config/boot.rb

@@ -1,3 +1,4 @@
 ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
 
 require "bundler/setup" # Set up gems listed in the Gemfile.
+require "bootsnap/setup"