fix: preserve workspace node_modules in Docker for hoisted mode

With hoisted mode, each workspace needs its own node_modules folder
(containing symlinks). Only ignore root /node_modules.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.dockerignore

@@ -1,6 +1,7 @@
 # Ignore development files
-node_modules
-**/node_modules
+# NOTE: With hoisted mode, we need the workspace node_modules symlinks
+# Only ignore root node_modules
+/node_modules
 .next
 .git
 .github