diff --git a/docs/reference/builder.md b/docs/reference/builder.md
index f8ef9cf56b..9ab4ba4df9 100644
--- a/docs/reference/builder.md
+++ b/docs/reference/builder.md
@@ -1306,11 +1306,18 @@ Keep the following things in mind about volumes in the `Dockerfile`.
 
 ## USER
 
-    USER daemon
+    USER <user>[:<group>]
+or
+    USER <UID>[:<GID>]
+
+The `USER` instruction sets the user name (or UID) and optionally the user
+group (or GID) to use when running the image and for any `RUN`, `CMD` and
+`ENTRYPOINT` instructions that follow it in the `Dockerfile`.
+
+> **Warning**:
+> When the user does doesn't have a primary group then the image (or the next
+> instructions) will be run with the `root` group.
 
-The `USER` instruction sets the user name or UID to use when running the image
-and for any `RUN`, `CMD` and `ENTRYPOINT` instructions that follow it in the
-`Dockerfile`.
 
 ## WORKDIR