Calling Python scripts from Java. Should I use Docker?
You don’t need docker for this. There are a couple of options, you should choose depending on what your Java application is doing. 

If the Java application is a client - based on swing, weblaunch, or providing UI directly - you will want to turn the python functionality to be wrapped in REST/HTTP calls. 
If the Java application is a server/webapp - executing within Tomcat, JBoss or other application container - you should simply wrap the python scrip inside a exec call. See the Java Runtime and ProcessBuilder API for this purpose.
We have a Java application in our project and what we want is to call some Python script and return results from it. What is the best way to do this?
We want to isolate Python execution to avoid affecting Java application at all. Probably, Dockerizing Python is the best solution. I don't know any other way.
Then, a question is how to call it from Java.
As far as I understand there are several ways:

start some web-server inside Docker which accepts REST calls from Java App and runs Python scripts and returns results to Java via REST too.
handle request and response via Docker CLI somehow.
use Java Docker API to send REST request to Docker which then converted by Docker to Stdin/Stdout of Python script inside Docker.

What is the most effective and correct way to connect Java App with Python, running inside Docker?