swh-apps: Check for dockefile existence in the right context

This also adapts to use the proper check as the previous check implemented always
returned false (without an absolute path that is).

Refs. swh/infra/sysadm-environment#4524

jobs/templates/swh-apps-build-docker-image.groovy.j2

@@ -4,7 +4,6 @@ def cleanup_image = params.CLEANUP_IMAGE
 
 def frozen_set_filepath = "apps/${application}/requirements-frozen.txt"
 def dockerfile_filepath = "apps/${application}/Dockerfile"
-def dockerfile_exists = new File(dockerfile_filepath).exists()
 
 // The following is configured in jenkins manually:
 // > Manage Jenkins > Configure System > Declarative pipeline (Docker)
@@ -121,6 +120,7 @@ pipeline {
             git checkout master
           """
           unstash "${application}-requirements-frozen"
+          dockerfile_exists = fileExists(dockerfile_filepath)
           if ( dockerfile_exists ) {
             commit_msg = "'${application}-${image_version}: Build image with frozen set requirements'"
           } else {