Added check if GID already exists

[clemenstyp]

I wanted to use borgwarehouse with group-id 100. But the build process failed because a group with that id already exist. therefore I added a small if clause to check if a group with that id already exists.

[Ravinou]

After thinking about it, I don't think I'll include your modification. In general, Dockerfiles are used to declaratively define an image, and conditional checks are often handled outside the context of the Docker image itself, for example in initialization scripts or orchestrators like Kubernetes. I don't think including this complexity in the dockerfile is a good idea.

But thanks for your suggestion all the same and for your interest in BorgWarehouse :)

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