Add clarification when using USER UID:GI

Signed-off-by: Michael Irwin <mikesir87@gmail.com>
(cherry picked from commit b3a1681fd81cae1abe67b1a2c0ad6f137593c6ea)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 365bae4f89a47841616d7aa087f1de68137bfaaa
Component: cli

Author: mikesir87

components/cli/docs/reference/builder.md

@@ -1484,6 +1484,9 @@ 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.