Please allow forks to inherit secure variables

Description

From https://disqus.com/home/discussion/gitlab-docs/group_level_variables_api_gitlab_documentation/#comment-3603674982

Proposal

We use variables to login into docker repository and provide some sensitive information like private keys. For any new fork of repository we enforced manually copy/paste it to allow CI work in forks too.

Please allow something like setting on variable "available on forks".