feat: export the devcontainer environment to allow docker exec to work

This will allow Dokku developers access to the devcontainer on their preferred terminal vs just the built-in one in VSCode.

.devcontainer/devcontainer.json

@@ -31,6 +31,7 @@
   ],
   "overrideCommand": false,
   "postCreateCommand": "setup-dev-env",
+  "postStartCommand": "printenv > .vscode/devcontainer.env",
   "runArgs": [
     "--init"
   ],