Add docs for VS Code Marketplace settings vars

MR: Pending

Update 2025-01-24

Since we are not supporting the Settings Module env vars for non-dev/test environments anymore as of !160133 (diffs) (i.e. they don't even work in production env), then we need to revisit this issue, and rewrite it to not make any mention of ENV vars.

Update 2024-05-09

With the merge of Add early adopter docs for VSCode extensions ga... (gitlab-org/remote-development/gitlab-remote-development-docs!51 - merged), we have a temporary placeholder of this documentation for early adopters, which includes references to the feature flags.

This doc is available at https://gitlab.com/gitlab-org/remote-development/gitlab-remote-development-docs/-/blob/main/doc/vscode-extensions-gallery.md

However, this issue remains open to represent the work needed to update the official documentation related to the extensions gallery, as well as potentially also adding documentation for all ENV vars supported by the Remote Development Settings Module.

Description

We need to decide where these docs should live. Considerations:

• Will they apply to both Web IDE and Workspaces?
• Do they need to be listed on https://docs.gitlab.com/ee/administration/environment_variables.html#supported-environment-variables?

We should also document all other variables from ee/lib/remote_development/settings/default_settings.rb while we are at it.

We should also consider mentioning https://docs.gitlab.com/16.10/omnibus/settings/environment-variables.html, which is the mechanism by which most non-local/non-GDK installations should set these ENV vars.

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As a [user or stakeholder], I want [goal or objective] so that [reason or benefit].

Acceptance Criteria

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• Also add documentation for all other existing variables in ee/lib/remote_development/settings/defaults_initializer.rb, e.g. GITLAB_REMOTE_DEVELOPMENT_PROJECT_CLONER_IMAGE
• [Describe what must be achieved to complete this issue.]
• [Describe another requirement needed to complete this issue.]
• [Add additional acceptance criteria as needed.]

Technical Requirements

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Placeholder notes for technical requirements on env vars:

• GITLAB_REMOTE_DEVELOPMENT_VSCODE_EXTENSIONS_MARKETPLACE_SETTINGS as implemented in Add extensions gallery support to Remote Dev se... (!149344 - merged)
• GITLAB_REMOTE_DEVELOPMENT_VSCODE_EXTENSIONS_GALLERY as implemented in Add JSON support to Remote Dev Settings module (!148287 - merged)

Design Requirements

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Impact Assessment

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User Story

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