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 .
 RUN rails assets:precompile && rm -fr log
 
 
@@ -18,6 +19,7 @@ ENV GOVUK_APP_NAME=smart-answers
 
 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

@@ -5,6 +5,7 @@ ruby File.read(".ruby-version").chomp
 gem "rails", "7.0.4"
 
 gem "ast"
+gem "bootsnap", require: false
 gem "gds-api-adapters"
 gem "gds_zendesk"
 gem "govspeak"

Gemfile.lock

@@ -75,6 +75,8 @@ GEM
       rack (>= 0.9.0)
     binding_of_caller (1.0.0)
       debug_inspector (>= 0.0.1)
+    bootsnap (1.15.0)
+      msgpack (~> 1.2)
     brakeman (5.2.3)
     builder (3.2.4)
     byebug (11.1.3)
@@ -218,6 +220,7 @@ GEM
       minitest (>= 4, < 6)
     mocha (2.0.2)
       ruby2_keywords (>= 0.0.5)
+    msgpack (1.6.0)
     multi_xml (0.6.0)
     multipart-post (2.1.1)
     net-imap (0.3.4)
@@ -430,6 +433,7 @@ DEPENDENCIES
   ast
   better_errors
   binding_of_caller
+  bootsnap
   byebug
   gds-api-adapters
   gds_zendesk

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"