Fix relative path not found on production web server

changelogs/unreleased/ak-fix-loading-file.yml

+---
+title: Fix relative path not found on production web server
+merge_request: 47090
+author:
+type: fixed

lib/gitlab/ci/runner_instructions.rb

@@ -106,7 +106,7 @@ module Gitlab
       end
 
       def get_file(path)
-        File.read(path)
+        File.read(Rails.root.join(path).to_s)
       end
 
       def registration_token

spec/lib/gitlab/ci/runner_instructions_spec.rb

@@ -75,6 +75,13 @@ RSpec.describe Gitlab::Ci::RunnerInstructions do
       with_them do
         let(:params) { { os: os, arch: arch } }
 
+        around do |example|
+          # puma in production does not run from Rails.root, ensure file loading does not assume this
+          Dir.chdir(Rails.root.join('tmp').to_s) do
+            example.run
+          end
+        end
+
         it 'returns string containing correct params' do
           result = subject.install_script