Dockerfile: Avoid rebuilding js in python step (#760)
Dockerfile: Avoid rebuilding js in python step (#760)

Set `TOX_ENV` to avoid re-running the javascript
generation step that has already been handled by
the nodejs `jsBuilder` step.

This causes the `NpmBuildCommand` inside
`setup.py` to abort early.

This fixes the Docker build as previously it would
fail to compile javascript in a pure-python docker
container.

Fix #759
posativ/isso,posativ/isso,posativ/isso,posativ/isso