docs/reference/builder.md: mention that USER directive also allows to specify the user group.
@@ -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
