fix(process-discovery): export containerId from docker module

The tracer_metadata module needs containerId from the docker module.
Export it explicitly to satisfy the eslint-require-export-exists rule.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tlhunter

packages/dd-trace/src/exporters/common/docker.js

@@ -35,6 +35,7 @@ if (inodePath) {
 const entityId = containerId ? `ci-${containerId}` : inode && `in-${inode}`
 
 module.exports = {
+  containerId,
   entityId,
 
   inject (carrier) {