Docker logstash file output problem

I have a problem with docker logstash (latest image) when I want to output logs to file:

- Logstash logs:

```
11:52:46.096 [[main]>worker0] INFO logstash.outputs.file - Opening file {:path=>"/etc/logstash/logs/log.log"}
11:52:46.279 [LogStash::Runner] FATAL logstash.runner - An unexpected error occurred! {:error=>#, :backtrace=>["org/jruby/RubyFile.java:370:in `initialize'", "org/jruby/RubyIO.java:871:in `new'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-file-4.0.1/lib/logstash/outputs/file.rb:280:in `open'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-file-4.0.1/lib/logstash/outputs/file.rb:132:in `multi_receive_encoded'", "org/jruby/RubyHash.java:1342:in `each'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-file-4.0.1/lib/logstash/outputs/file.rb:131:in `multi_receive_encoded'", "org/jruby/ext/thread/Mutex.java:149:in `synchronize'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-file-4.0.1/lib/logstash/outputs/file.rb:130:in `multi_receive_encoded'", "/usr/share/logstash/logstash-core/lib/logstash/outputs/base.rb:90:in `multi_receive'", "/usr/share/logstash/logstash-core/lib/logstash/output_delegator_strategies/shared.rb:12:in `multi_receive'", "/usr/share/logstash/logstash-core/lib/logstash/output_delegator.rb:43:in `multi_receive'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:336:in `output_batch'", "org/jruby/RubyHash.java:1342:in `each'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:335:in `output_batch'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:293:in `worker_loop'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:263:in `start_workers'"]}
```

- Here is my output config:

`output {
file {
path => [ "/etc/logstash/logs/log.log" ]
}
}
`

Can someone help me?