Restic/builder still relies on vendor directory

As far as I can tell https://github.com/restic/builder still relies on go modules being present in the vendor directory:

$ docker run --rm \
    --volume "$(pwd):/restic" \
    --volume "$(pwd)/output:/output" \
    restic/builder
build github.com/spf13/pflag: cannot load github.com/spf13/pflag: open /restic/vendor/github.com/spf13/pflag: no such file or directory

Does anybody know how to create reproducible builds with restic builder?