Make gdk.yml single source of truth

Overview

Currently, configuration can be set via:

• gdk.yml
• env.runit
• hardcoded in service classes (example)

Let's make gdk.yml single source of truth.

Implementation plan

• Allow setting global environment variables: config: Define global environment variables (!4206 - merged)
• Implement Service::Base#global_env to merge with config.env
• Document the use of config.env, Services::Base#env, Services::Base#global_env
• Deprecate env.runit
• Remove file based secret storage like gitlab-pages-secret

Impacted categories

The following categories relate to this issue:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Steps to replicate (optional)

Proposal (optional)

Environment (optional)

• Operating system name: <!-- output of `uname -a` command -->
• Architecture: <!-- output of `arch` command -->
• The contents of your gdk.yml (if any)
• Ruby version: <!-- output of `ruby --version` command -->
• GDK version: <!-- output of `git rev-parse --short HEAD` command -->