Dockerfile: Avoid rebuilding js in python step

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

Dockerfile

@@ -7,6 +7,7 @@ RUN make init js
 # Second, create virtualenv
 FROM python:3.8-buster AS venvBuilder
 WORKDIR /src/
+ENV TOX_ENV=1
 COPY --from=jsBuilder /src .
 RUN python3 -m venv /isso \
  && . /isso/bin/activate \