nsjail author here (the original one, as the tool is also maintained by others), good job!
Irrelevant nit: .proto files are protobuf definition files (like this one: https://github.com/google/nsjail/blob/master/config.proto), a text representation of a specific protobuf contents is typically called (as per man clang-format): .textpb .pb.txt or .textproto - I use .config for examples distributed with nsjail, but it's licentia poetica :)