Consolidate duplicated user selection logic into shared helper

[Copilot]

18 feature scripts contained identical 20-line blocks for determining the non-root user during installation. This creates maintenance burden and drift risk.

Changes

• New helper: src/_common/common-setup.sh with determine_user_from_input() function
• Migration: Replaced inline logic in 18 features with 4-line helper call
• Tests: Added test/_common/test-common-setup.sh covering edge cases
• Docs: Added src/_common/README.md with API and migration guide

Implementation

The helper consolidates user detection priority:

1. _REMOTE_USER env var (if set and not root, with existence validation)
2. Priority list: devcontainer → vscode → node → codespace → UID 1000
3. Configurable fallback (typically root or vscode)

# Before (20+ lines per feature)
if [ "${USERNAME}" = "auto" ] || [ "${USERNAME}" = "automatic" ]; then
    USERNAME=""
    POSSIBLE_USERS=("vscode" "node" "codespace" "$(awk -v val=1000 -F ":" '$3==val{print $1}' /etc/passwd)")
    for CURRENT_USER in "${POSSIBLE_USERS[@]}"; do
        if id -u ${CURRENT_USER} > /dev/null 2>&1; then
            USERNAME=${CURRENT_USER}
            break
        fi
    done
    if [ "${USERNAME}" = "" ]; then
        USERNAME=root
    fi
elif [ "${USERNAME}" = "none" ] || ! id -u ${USERNAME} > /dev/null 2>&1; then
    USERNAME=root
fi

# After (4 lines)
SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
source "${SCRIPT_DIR}/../_common/common-setup.sh"
USERNAME=$(determine_user_from_input "${USERNAME}" "root")

Notable Details

• common-utils special case: Preserved USERNAME="none" → USER_UID=0/USER_GID=0 behavior by checking original input before transformation
• _REMOTE_USER validation: Added existence check to prevent invalid usernames from propagating
• Backward compatible: All existing behavior preserved; features updated: anaconda, common-utils, conda, desktop-lite, docker-in-docker, docker-outside-of-docker, go, hugo, java (+ wrapper.sh), kubectl-helm-minikube, node, oryx, php, python, ruby, rust, sshd

Net: -188 lines of duplicated logic, +87 lines shared implementation (excluding tests/docs).

Original prompt

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This section details on the original issue you should resolve

<issue_title>Avoid duplicating user selection logic in features</issue_title>
<issue_description>Multiple scripts have similar / the same logic for determining which user should be selected for feature installation. We need to unify this to avoid duplication and annoying issues down the line.
https://github.com/devcontainers/features/pull/1530/changes#diff-26d9422c201004aa5880edd23a05e8e5cd69c42a2395391dd23b4ab2a62aa26bR246-R264</issue_description>

<agent_instructions>Please find all occurrences where this logic is used and unify it into a helper script. Ensure that all edge-cases are considered, and tests are written for the helper (not needed for the other scripts).</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Avoid duplicating user selection logic in features #1551

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