Use gosu for reliable UID:GID switching

Fix PUID/PGID collision issues by using gosu to run services with exact UID:GID. Changes include:

- Added redis-start.sh and updated app-start.sh to load /etc/environment, determine PUID/PGID, and invoke gosu "$PUID:$PGID" to start Redis and the Next.js app (with verification and fallbacks).
- Updated entrypoint.sh to persist PUID/PGID into /etc/environment, document the gosu approach, and adjust startup messaging.
- Updated supervisord.conf to run the new startup wrappers as root (so they can use gosu) instead of running processes directly as specific users.
- Dockerfile updated to install gosu and copy the redis-start.sh wrapper.
- Documentation updated (deployment/unified.md) describing the PUID collision bug, the root cause, and the gosu-based fix.

This resolves cases where PUID collides with existing system users (e.g., nobody) which previously caused processes to run with the wrong GID and produce EACCES errors.

docker/unified/redis-start.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+# Redis startup wrapper for unified container
+# Uses gosu to ensure correct PUID:PGID for file operations
+
+set -e
+
+# Load environment from /etc/environment (set by entrypoint)
+if [ -f /etc/environment ]; then
+    set -a
+    source /etc/environment
+    set +a
+fi
+
+# Get PUID/PGID (default to redis user's current IDs if not set)
+PUID=${PUID:-$(id -u redis)}
+PGID=${PGID:-$(id -g redis)}
+
+echo "[Redis] Starting Redis server..."
+echo "[Redis] Process will run as UID:GID = $PUID:$PGID"
+
+# Use gosu to switch to correct UID:GID and start redis
+# This bypasses username resolution issues when PUID collides with existing users
+if [ "$(id -u)" = "0" ]; then
+    # Running as root - use gosu to switch to PUID:PGID
+    echo "[Redis] Switching to UID:GID $PUID:$PGID via gosu..."
+    exec gosu "$PUID:$PGID" /usr/bin/redis-server --appendonly yes --dir /var/lib/redis --bind 127.0.0.1 --port 6379
+else
+    # Not running as root - just run directly (fallback)
+    echo "[Redis] Warning: Not running as root, cannot use gosu. Running as current user."
+    exec /usr/bin/redis-server --appendonly yes --dir /var/lib/redis --bind 127.0.0.1 --port 6379
+fi