diff --git a/.dockerignore b/.dockerignore
index 17e5e297..3c505b35 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -43,9 +43,11 @@ Thumbs.db
 *.md
 !README.md
 # The feature vault is data, not documentation: /roadmap/graph reads these notes
-# at runtime, so they have to survive the *.md filter above.
+# at runtime, so they have to survive the *.md filter above. Keep this as the
+# bare directory — Railpack turns each negation into a literal copy instruction,
+# and a `!vault/**` glob becomes `copy /vault/** /app/**`, which buildkit rejects
+# with "cannot copy to non-directory". `!vault` alone copies the tree recursively.
 !vault
-!vault/**
 docs
 
 # Docker