Optimize docker image

* Use 2 stage docker building to produce a tiny python3-slim based docker image with just the compilation results.

package.json

@@ -17,6 +17,9 @@
     "url": "https://github.com/maputnik/editor"
   },
   "author": "Lukas Martinelli",
+  "contributors": [
+    "Yuri Astrakhan <YuriAstrakhan@gmail.com>"
+  ],
   "license": "MIT",
   "homepage": "https://github.com/maputnik/editor#readme",
   "dependencies": {