fix(docker): fix bugs and refactor Docker entrypoint script

[ilyam8]

Summary

This PR fixes several bugs in the Docker entrypoint script and refactors group management logic for better maintainability.

• Fixes

Bug	Description
--apend typo	usermod --apend silently failed, preventing users from being added to the Proxmox config files group
Group membership check	grep -q "${DOCKER_USR}" matched substrings, causing false positives (e.g., user net incorrectly detected as member when netdata was in the group)
Telemetry disable logic	Setting DISABLE_TELEMETRY=0 incorrectly disabled telemetry because -n "$VAR" is true for any non-empty string including "0"
Unconditional export	PGID and DOCKER_HOST were exported even when empty
Missing default	NETDATA_LISTENER_PORT had no fallback value (now defaults to 19999)

• Refactoring

Change	Description
is_user_in_group()	New helper function for proper group membership checking
add_user_to_gid()	New helper function to eliminate duplicate group creation/assignment logic
Variable naming	Consistent group_gid instead of mixed group_guid/group_gid

Test Plan

Additional Information

For users: How does this change affect me?

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Summary by cubic

Fixes multiple issues in the Docker entrypoint to make group assignment and telemetry control reliable, and sets a default Netdata listener port. Refactors group management into small helpers to reduce duplication and improve maintainability.

• Bug Fixes

  • Corrected usermod --append and added precise GID-based group membership check (no substring matches).
  • Fixed telemetry opt-out to treat "0" as enabled; export PGID/DOCKER_HOST only when set.
  • Defaulted NETDATA_LISTENER_PORT to 19999; minor robustness tweaks (error redirection, container hostname write).

• Refactors

  • Added is_user_in_group and add_user_to_gid helpers and reused them for Docker, Proxmox, and NVIDIA group setup.
  • Standardized variable names (group_gid) and simplified early returns for root/no-GID cases.

^{Written for commit 77344f9. Summary will update automatically on new commits.}

[Copilot]

Pull request overview

This PR fixes critical bugs in the Docker entrypoint script and refactors group management logic for better maintainability and reliability.

Key Changes:

• Fixes the --apend typo (now --append) that prevented users from being added to groups
• Corrects group membership detection using exact matching instead of substring matching
• Fixes telemetry disable logic to properly handle DISABLE_TELEMETRY=0
• Introduces reusable helper functions is_user_in_group() and add_user_to_gid() to eliminate code duplication

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[cubic-dev-ai]

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