[Python] Importing a Python dependency from a private repository causes docker buildx build on...

I have setup a Kubernetes pod to build Docker containers. The projects I'm building containers for have a Python dependency which is from a private repository. Here is the issue: if I run the docker buildx build ... command in my terminal, everything works. If, however, I run the command through a Python scripts, it fails with

#1 ERROR: error for bootstrap "kube30": Unauthorized
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> [internal] booting buildkit:
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ERROR: error for bootstrap "kube30": Unauthorized


Note that it only fails if I explicitly import the private dependency. If I remove the import, without changing anything else, the building process works. And again, only when going through a script: when using the command directly from cmdline, it works in either case.

What could be the problem?

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