Create a CSV file with column names in the first row.
One column must contain the user_id (not username). Custom user fields have internal names like user_field_1, so the other columns need to have those as column names.
Move the user_fields.rake file into /var/www/discourse/lib/tasks inside the container (perhaps by putting it in /var/discourse/shared/standalone outside the container and then moving it from inside the container).
You can then
rake user_fields:import_csv[csv-filename.csv]
And the fields from the CSV will be updated or created in the user records.
This script is available free and is unsupported. If you would like to pay for support, feel free to contact me.