Fix 500 error in CI lint when included templates are an array

Previously the following syntax would fail in the linter with an
error 500:

```
include:
  template:
    - License-Management.gitlab-ci.yml
    - Dependency-Scanning.gitlab-ci.yml
    - SAST.gitlab-ci.yml
```

Now the error will call out specifically that the value
is not a string.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66605

changelogs/unreleased/sh-fix-ci-lint-500-error.yml

+---
+title: Fix 500 error in CI lint when included templates are an array
+merge_request: 32232
+author:
+type: fixed

doc/ci/yaml/README.md

@@ -2170,6 +2170,14 @@ include:
   - template: Auto-DevOps.gitlab-ci.yml
 ```
 
+Multiple `include:template` files:
+
+```yaml
+include:
+  - template: Android-Fastlane.gitlab-ci.yml
+  - template: Auto-DevOps.gitlab-ci.yml
+```
+
 All [nested includes](#nested-includes) will be executed only with the permission of the user,
 so it is possible to use project, remote or template includes.
 

lib/gitlab/ci/config/external/file/base.rb

@@ -26,6 +26,10 @@ module Gitlab
               location.present?
             end
 
+            def invalid_location_type?
+              !location.is_a?(String)
+            end
+
             def invalid_extension?
               location.nil? || !::File.basename(location).match?(YAML_WHITELIST_EXTENSION)
             end
@@ -71,7 +75,9 @@ module Gitlab
             end
 
             def validate_location!
-              if invalid_extension?
+              if invalid_location_type?
+                errors.push("Included file `#{location}` needs to be a string")
+              elsif invalid_extension?
                 errors.push("Included file `#{location}` does not have YAML extension!")
               end
             end

spec/lib/gitlab/ci/config/external/file/base_spec.rb

@@ -41,6 +41,12 @@ describe Gitlab::Ci::Config::External::File::Base do
   end
 
   describe '#valid?' do
+    context 'when location is not a string' do
+      let(:location) { %w(some/file.txt other/file.txt) }
+
+      it { is_expected.not_to be_valid }
+    end
+
     context 'when location is not a YAML file' do
       let(:location) { 'some/file.txt' }