code: function createHeader (type, size) {
    var header = new Buffer(8);
    header.writeUInt8(type, 0);
    header.writeUInt32BE(size, 4);
    return header;
  }
docs: create a docker frame header
  @param {number} ioConnectionType - 2=stderr, 1=stdout, 0=stdin
  @param {string|buffer} payloadData
  @returns {buffer} header - frame header (type, length) as buffer