Description

The Debian installer script currently detects contrib repositories by grepping apt-cache policy output for strings like bookworm-updates/contrib. This approach is brittle and produces false negatives on Debian 12 systems using modern APT configurations, especially:

• deb822 sources (/etc/apt/sources.list.d/*.sources)
• mirror+file:///etc/apt/mirrors/*.list mirror lists

As a result, the installer repeatedly warns that bookworm-updates/contrib is missing even when Components: ... contrib ... is correctly configured for bookworm, bookworm-updates, and bookworm-security.

This change replaces the fragile apt-cache policy string matching with a more robust detection strategy:

• Primary: parse apt-get indextargets (APT’s effective index target list) to confirm Component: contrib for the relevant suite/codename.
• Support Debian suite aliases commonly present in Release metadata (stable, stable-updates, stable-security) in addition to codename suites.
• Fallbacks: parse deb822 *.sources stanzas and classic sources.list / *.list formats if needed.

Behavior is unchanged in terms of requirements:

• contrib for the base suite remains required
• updates/contrib and security/contrib remain recommended (warn + prompt)

Dependencies: none.

Fixes # (issue title here)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Reproduced and verified on Debian 12 (Bookworm) using deb822 sources with mirror lists:

Example /etc/apt/sources.list.d/*.sources stanza:

Types: deb deb-src
URIs: mirror+file:///etc/apt/mirrors/debian.list
Suites: bookworm bookworm-updates bookworm-backports
Components: main contrib non-free non-free-firmware
Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg

Steps

1. Configure Debian 12 APT sources using deb822 .sources and mirror+file mirror lists including contrib.
2. Run apt update (ensure indexes refresh successfully).
3. Run the installer script.

Expected Results

• If bookworm-updates includes contrib, the installer should NOT print the warning:
  Missing bookworm-updates/contrib repository.

• If bookworm-updates does NOT include contrib, the installer should still print the warning and prompt to continue.

• Docker

• Other (Debian 12 Bookworm, deb822 .sources, mirror+file mirror lists)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have tested that my fix is effective or that my feature works

Changelog:

• Replace contrib repo detection logic with parsing of apt-get indextargets (and fallbacks) to avoid false negatives.
• Accept Debian suite aliases (stable, stable-updates, stable-security) in addition to codename suites.