Add clarification when using USER UID:GI
@@ -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.
