groupmod: GID '999' already exists

[aliceinwire]

Probably the following task should be conditional if 999 doesn't exist already

 => CACHED [worker 14/28] RUN mkdir -p /var/www/fileserver                                            0.0s
 => CACHED [worker 15/28] RUN chown -R buildbot /var/www/fileserver                                   0.0s
 => ERROR [worker 16/28] RUN groupmod -g 999 docker                                                   0.3s
 => [cron] resolving provenance for metadata file                                                     0.0s
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 > [worker 16/28] RUN groupmod -g 999 docker:
0.277 groupmod: GID '999' already exists
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failed to solve: process "/bin/sh -c groupmod -g $DOCKER_GID docker" did not complete successfully: exit code: 4