fix: do not create group if GID is already taken

macOS uses GID 20 for staff but Linux uses it for dialout, so the build
will fail because GID 20 is already used.

Author: kwvg

contrib/containers/ci/ci-slim.Dockerfile

@@ -106,7 +106,7 @@ RUN set -ex; \
 ARG USER_ID=1000 \
     GROUP_ID=1000
 RUN set -ex; \
-    groupmod -g ${GROUP_ID} -n dash ubuntu; \
+    getent group ${GROUP_ID} || groupmod -g ${GROUP_ID} -n dash ubuntu; \
     usermod -u ${USER_ID} -md /home/dash -l dash ubuntu; \
     chown ${USER_ID}:${GROUP_ID} -R /home/dash; \
     mkdir -p /src/dash && \