Reduce images size (#1636)

## Reduce image size

~:warning: Depends on #1635

Reduce Kuzzle images size:
 - `kuzzleio/kuzzle:2`: from 563 Mo to 165 Mo
 - `kuzzleio/plugin-dev:2`: from 968 Mo to 410 Mo

This PR also includes 2 new tags:
 - `kuzzleio/kuzzle:2-alpine`: 140 Mo
 - `kuzzleio/kuzzle:2-scratch`: 94 Mo

I followed the tricks explained here: https://medium.com/trendyol-tech/how-we-reduce-node-docker-image-size-in-3-steps-ff2762b51d5a

Also I made few other changes for the production image:
 - remove all object and build files (`.o`, `.h`)
 - add only relevant files (`lib`, `config`, `plugins`, etc)
 - remove `.git`
 - remove useless npm dependencies
 - minify JS
 - strip `re2` and `node` binaries

The `scratch` image contains only Node.js (with libstdc++), musl and Kuzzle source code.

## Scripts

All docker related scripts are in `docker/scripts/`:

 - `entrypoint.sh`: the entrypoint
 - `clean-node.sh`: remove every unnecessary file from a directory. If `NODE_ENV=production` then the JS and JSON files will be minified
 - `run-dev.sh`: used by the `core-dev` image to run a core-developer stack
 - `run-plugin.sh`: used by `plugin-dev` to run an application composed of plugins (like we do now)
 - `run-prod.sh`: used by the `kuzzle` images to run kuzzle in production

### How should this be manually tested?

Build the images and play the functional tests

### Other changes

 - Elasticsearch initialization is now waited with the `bin/wait-elasticsearch` script instead of curl
 - Functional tests are run in parallel in 2 travis jobs

Author: Aschen

.babelrc

@@ -68,11 +68,11 @@ ENV NODE_ENV=production
 
 COPY --from=dependencies /var/app/ /var/app/
 
-RUN  ln -s /var/app/docker-compose/scripts/run.sh /usr/local/bin/kuzzle \
+RUN  ln -s /var/app/docker/scripts/run.sh /usr/local/bin/kuzzle \
   && chmod a+x /usr/local/bin/kuzzle \
-  && chmod a+x /var/app/docker-compose/scripts/docker-entrypoint.sh
+  && chmod a+x /var/app/docker/scripts/docker-entrypoint.sh
 
-ENTRYPOINT ["/var/app/docker-compose/scripts/docker-entrypoint.sh"]
+ENTRYPOINT ["/var/app/docker/scripts/docker-entrypoint.sh"]
 
 CMD ["kuzzle", "start"]