Update devcontainer.json

Author: mikaem

.devcontainer/devcontainer.json

@@ -5,7 +5,9 @@
   },
   "waitFor": "onCreateCommand",
   "updateContentCommand": "conda env create --file environment.yml",
-  "postCreateCommand": "",
+  "postCreateCommand": ["source activate shenfun", 
+                        "python setup.py build_ext -i", 
+                        "export PYTHONPATH=$PWD"],
   "customizations": {
     "codespaces": {
       "openFiles": []