Make docker utils work with psych 4

Change-Id: I958a41f6cd2a943fe98f66012c90d01da24c51b5
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/307803
Reviewed-by: Aaron Ogata <aogata@instructure.com>
Build-Review: Aaron Ogata <aogata@instructure.com>
QA-Review: Jacob Burroughs <jburroughs@instructure.com>
Product-Review: Jacob Burroughs <jburroughs@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>

build/docker_utils.rb

@@ -33,7 +33,11 @@ class DockerUtils
       end
 
       compose_files.inject({}) do |config, file|
+        if YAML::VERSION < "4.0"
           config.merge(YAML.load_file(file), &merger)
+        else
+          config.merge(YAML.load_file(file, aliases: true), &merger)
+        end
       end
     end
   end