Merge branch 'dockerfile-templates' into 'master'

app/assets/javascripts/api.js

app/assets/javascripts/blob/blob_dockerfile_selector.js.es6

app/assets/javascripts/blob/blob_dockerfile_selectors.js.es6

app/assets/javascripts/blob_edit/edit_blob.js

app/assets/stylesheets/pages/editor.scss

app/helpers/blob_helper.rb

app/views/projects/blob/_editor.html.haml

changelogs/unreleased/dockerfile-templates.yml

lib/api/templates.rb

lib/gitlab/template/dockerfile_template.rb

spec/features/projects/files/dockerfile_dropdown_spec.rb

vendor/dockerfile/HTTPdDockerfile