diff --git a/docs/reference/builder.md b/docs/reference/builder.md
index 2b24052255..3e19e10e55 100644
--- a/docs/reference/builder.md
+++ b/docs/reference/builder.md
@@ -1478,6 +1478,9 @@ 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`.
 
+> Note that when specifying a group for the user, the user will have _only_ the
+> specified group membership. Any other configured group memberships will be ignored.
+
 > **Warning**:
 > When the user doesn't have a primary group then the image (or the next
 > instructions) will be run with the `root` group.