fix: Skip groupadd if gid exists in Docker setup

[vidbina]

Users who have a gid that matching an existing gid in the container will encounter an error when running groupadd. In order to allow the docker build process to continue, the addition of the -f flag will allow groupadd to exit successfully if the group already exists.

[SafariMonkey]

I'm not convinced that this is the best solution. I'd like to point out the following sentence from the documentation:

When used with -g, and the specified GID already exists, another (unique) GID is chosen (i.e. -g is turned off).

This means that it will create a group with that name but a different GID unnecessarily.

If the GID exists, the next line will run without an issue. If it doesn't, it'll presumably fail, based on the note:

A group number must refer to an already existing group.

Based on this, I suggest, as a possibility, just adding || true to the previous line. The command output will still show in the console if it fails, and if the next line succeeds, all is well.

-f should be fine, of course, as the extra group is only being created inside the container. This just seems like a cleaner solution to me.

[vidbina]

Good spot, @SafariMonkey. Based it from the groupadd --help in the base image which mentioned the following for -f:

exit successfully if the group already exists, and cancel -g if the GID is already used

I understood this bit to mean that the use of -f with -g would be "safe enough".

└──> docker run --rm -it ubuntu:16.04 bash
root@8e4d4c8550cd:/# groupadd --help
Usage: groupadd [options] GROUP

Options:
  -f, --force                   exit successfully if the group already exists,
                                and cancel -g if the GID is already used
  -g, --gid GID                 use GID for the new group
  -h, --help                    display this help message and exit
  -K, --key KEY=VALUE           override /etc/login.defs defaults
  -o, --non-unique              allow to create groups with duplicate
                                (non-unique) GID
  -p, --password PASSWORD       use this encrypted password for the new group
  -r, --system                  create a system account
  -R, --root CHROOT_DIR         directory to chroot into
      --extrausers              Use the extra users database

Did look into the resulting image and notice that the builder group was created with a new gid (999 in my case), so I have updated the PR such that we don't have this rogue group in the the image. Thanks for your input 🙏

@canselcik let me know if this is worthwhile merging or if you'd like me to supply this PR squashed.