Send a transaction using bitcoinjs-lib to testnet in a box
The propagate function isn't designed for testnet-in-a-box because there is no routing for requests in the bitoind program that testnet-in-a-box activates.
Rather testnet-in-a-box is designed for JSON-RPC, so you can use sendrawtransaction, which described in https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_calls_list.

This is the propagate function:

blockchain.transactions.propagate = function broadcast (txHex, callback) {
  dhttp({
    method: 'POST',
    url: 'https://api.ei8ht.com.au:9443/3/pushtx',
    body: txHex
  }, callback)
}