chore(docker/.env.example): Add TOP_K_MAX_VALUE to the `.env.example`… (#12422)

Signed-off-by: -LAN- <laipz8200@outlook.com>

docker/.env.example

 # Maximum number of submitted thread count in a ThreadPool for parallel node execution
 MAX_SUBMIT_COUNT=100
 
+# The maximum number of top-k value for RAG.
+TOP_K_MAX_VALUE=10

docker/docker-compose.yaml

   CSP_WHITELIST: ${CSP_WHITELIST:-}
   CREATE_TIDB_SERVICE_JOB_ENABLED: ${CREATE_TIDB_SERVICE_JOB_ENABLED:-false}
   MAX_SUBMIT_COUNT: ${MAX_SUBMIT_COUNT:-100}
+  TOP_K_MAX_VALUE: ${TOP_K_MAX_VALUE:-10}
 
 services:
   # API service