Don't use the root group (#690)

Summary:
**Pre-submission checklist**
- [x] I've ran the linters locally and fixed lint errors related to the
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In commit 7b36e2f (#641) I mistakenly added the user to the `root`
group, which is not necessary and gives more privileges than we want to
have.

Pull Request resolved: #690

Test Plan: Not needed.

Reviewed By: shiva-menta

Differential Revision: D88080346

Pulled By: KevinYakar

fbshipit-source-id: 82b004ff866721729661a974a16ae064ab12af48

Author: benoit-nexthop

fboss/oss/docker/Dockerfile

@@ -116,7 +116,7 @@ RUN echo "%wheel ALL=(ALL) NOPASSWD:ALL" >> /etc/sudoers
 # Create user if USERNAME is not root
 RUN if [ "$USERNAME" != "root" ] && [ "$USER_UID" != "0" ]; then \
     groupadd -g $USER_GID $USERNAME && \
-    useradd -l -m -u $USER_UID -g $USER_GID -G root,adm,wheel,users -s $USER_SHELL $USERNAME && \
+    useradd -l -m -u $USER_UID -g $USER_GID -G adm,wheel,users -s $USER_SHELL $USERNAME && \
     chown -R $USERNAME /var/FBOSS; \
     fi