Bundler Version is not spezified in Docker Image

When building the docker image the Bundler Version gets wrong. 

The following error is shown:

```
Step 14/18 : RUN gem install bundler --no-rdoc --no-ri
 ---> Running in 9da643e9d66f
ERROR:  Error installing bundler:
	bundler requires Ruby version >= 2.3.0.
```