Use printenv when writing runtime env

CloudronManifest.json

@@ -5,7 +5,7 @@
   "description": "Next-gen knowledge base that blends docs, whiteboards, and databases for self-hosted teams.",
   "website": "https://affine.pro",
   "contactEmail": "support@affine.pro",
-  "version": "0.1.6",
+  "version": "0.1.7",
   "changelog": "Initial Cloudron packaging",
   "manifestVersion": 2,
   "minBoxVersion": "7.0.0",

start.sh

@@ -147,7 +147,7 @@ write_runtime_env() {
   )
   local var value
   for var in "${vars[@]}"; do
-    value="${!var-}"
+    value=$(printenv "$var" 2>/dev/null || true)
     if [ -n "$value" ]; then
       printf '%s=%q\n' "$var" "$value" >> "$ENV_EXPORT_FILE"
     fi