problem in docker build of lab

if i do a `git clean -fdx` before the docker build, then i get:
```

Errored, use --debug for full output:
ValueError: "/home/beakerx/js/lab" is not a valid extension:
Missing extension module "dist/index.js"
Missing mimeExtension module "dist/javascriptRendererExtension.js"
The command '/bin/sh -c /home/beakerx/setup.sh' returned a non-zero code: 1
```

if i build lab then it works. but really, the software should all be built in the container.
i think we should fix this and put the "clean" into the release doc.