fix: unstructured io credential environment variables missing (#10953)

docker/.env.example

 # `Unstructured` Unstructured.io file extraction scheme
 ETL_TYPE=dify
 
-# Unstructured API path, needs to be configured when ETL_TYPE is Unstructured.
+# Unstructured API path and API key, needs to be configured when ETL_TYPE is Unstructured
+# Or using Unstructured for document extractor node for pptx.
 # For example: http://unstructured:8000/general/v0/general
 UNSTRUCTURED_API_URL=
+UNSTRUCTURED_API_KEY=
 
 # ------------------------------
 # Model Configuration

docker/docker-compose.yaml

   UPLOAD_FILE_BATCH_LIMIT: ${UPLOAD_FILE_BATCH_LIMIT:-5}
   ETL_TYPE: ${ETL_TYPE:-dify}
   UNSTRUCTURED_API_URL: ${UNSTRUCTURED_API_URL:-}
+  UNSTRUCTURED_API_KEY: ${UNSTRUCTURED_API_KEY:-}
   PROMPT_GENERATION_MAX_TOKENS: ${PROMPT_GENERATION_MAX_TOKENS:-512}
   CODE_GENERATION_MAX_TOKENS: ${CODE_GENERATION_MAX_TOKENS:-1024}
   MULTIMODAL_SEND_IMAGE_FORMAT: ${MULTIMODAL_SEND_IMAGE_FORMAT:-base64}