Make sure /app is always in sys.path

[bcdickinson]

Description

Ensures /app/ in the Docker container is always on sys.path so we can import the tests, etc.

Fixes #233

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

[tbrlpld]

Works and looks good. Thanks @bcdickinson