Add DOCKERFILE_PATH to Auto DevOps workflow rules

In https://gitlab.com/gitlab-org/gitlab/-/issues/212211 we added
support for custom Dockerfile paths for Auto DevOps via the
DOCKERFILE_PATH CI variable, but the workflow rule detection only
looks for a Dockerfile at the project root.

changelogs/unreleased/add-dockerfile-path-to-ado-workflow-rules.yml

+---
+title: Add DOCKERFILE_PATH to Auto DevOps workflow:rules
+merge_request: 36475
+author:
+type: fixed

lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml

@@ -74,7 +74,7 @@ stages:
 
 workflow:
   rules:
-    - if: '$BUILDPACK_URL || $AUTO_DEVOPS_EXPLICITLY_ENABLED == "1"'
+    - if: '$BUILDPACK_URL || $AUTO_DEVOPS_EXPLICITLY_ENABLED == "1" || $DOCKERFILE_PATH'
 
     - exists:
         - Dockerfile

spec/lib/gitlab/ci/templates/auto_devops_gitlab_ci_yaml_spec.rb

@@ -205,6 +205,7 @@ RSpec.describe 'Auto-DevOps.gitlab-ci.yml' do
       'Buildpack'       | { 'README.md' => '' }                   | { 'BUILDPACK_URL' => 'http://example.com' } | %w(build test) | %w()
       'Explicit set'    | { 'README.md' => '' }                   | { 'AUTO_DEVOPS_EXPLICITLY_ENABLED' => '1' } | %w(build test) | %w()
       'Explicit unset'  | { 'README.md' => '' }                   | { 'AUTO_DEVOPS_EXPLICITLY_ENABLED' => '0' } | %w()           | %w(build test)
+      'DOCKERFILE_PATH' | { 'README.md' => '' }                   | { 'DOCKERFILE_PATH' => 'Docker.file' }      | %w(build test) | %w()
       'Dockerfile'      | { 'Dockerfile' => '' }                  | {}                                          | %w(build test) | %w()
       'Clojure'         | { 'project.clj' => '' }                 | {}                                          | %w(build test) | %w()
       'Go modules'      | { 'go.mod' => '' }                      | {}                                          | %w(build test) | %w()