Update code to validate python bin versions

[btalanski]

Python versions weren't being properly validated and displayed in the app.

• Updated the code to make validation work
• Update Config class with methods to read/check env vars
• Add python3 env var to dockerfile

[MrBrax]

Could you change the branch to develop?

The detection of python isn't all that important to be honest since it always uses the executable shortcuts instead, but nice patch