BE: re-implement APP_CONF_OVERRIDE support

Signed-off-by: jokob-sk <jokob.sk@gmail.com>

install/production-filesystem/entrypoint.d/31-apply-conf-override.sh

@@ -0,0 +1,35 @@
+#!/bin/sh
+# override-config.sh - Handles APP_CONF_OVERRIDE environment variable
+
+OVERRIDE_FILE="${NETALERTX_CONFIG}/app_conf_override.json"
+
+# Ensure config directory exists
+mkdir -p "$(dirname "$NETALERTX_CONFIG")" || {
+    >&2 echo "ERROR: Failed to create config directory $(dirname "$NETALERTX_CONFIG")"
+    exit 1
+}
+
+# Remove old override file if it exists
+rm -f "$OVERRIDE_FILE"
+
+# Check if APP_CONF_OVERRIDE is set
+if [ -z "$APP_CONF_OVERRIDE" ]; then
+    >&2 echo "APP_CONF_OVERRIDE is not set. Skipping override config file creation."
+else
+    # Save the APP_CONF_OVERRIDE env variable as a JSON file
+    echo "$APP_CONF_OVERRIDE" > "$OVERRIDE_FILE" || {
+        >&2 echo "ERROR: Failed to write override config to $OVERRIDE_FILE"
+        exit 2
+    }
+
+    RESET=$(printf '\033[0m')
+    >&2 cat <<EOF
+══════════════════════════════════════════════════════════════════════════════
+📝  APP_CONF_OVERRIDE detected. Configuration written to $OVERRIDE_FILE.
+
+    Make sure the JSON content is correct before starting the application.
+══════════════════════════════════════════════════════════════════════════════
+EOF
+
+    >&2 printf "%s" "${RESET}"
+fi