How to get field values in node template in Twig
A teaser image linked to the content is the standard configuration for the teaser view mode. You only need to limit it to one image. This is a feature request for core, see this issue https://www.drupal.org/node/1234624. Until then you can use this sandbox project: https://www.drupal.org/node/1234624#comment-11868691.

If you want to do this in a node twig you can get the first field item with the delta 0:

{{ content.field_image.0 }}


and then remove the field from the main content to avoid double content:

{{ content|without('field_image') }}