Expand instance-level templates to cover .gitlab-ci.yml, .gitignore and Dockerfile

What does this MR do?

In %11.3, we introduced a custom instance-level templates repository for license templates: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6631

This MR expands support for custom instance-level templates to three other types of file.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

What are the relevant issue numbers?

Closes #5306 (closed)

app/finders/template_finder.rb

 class TemplateFinder
+  prepend ::EE::TemplateFinder
+
   VENDORED_TEMPLATES = {
     dockerfiles: ::Gitlab::Template::DockerfileTemplate,
     gitignores: ::Gitlab::Template::GitignoreTemplate,