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

@@ -31,9 +31,10 @@
   ],
   "overrideCommand": false,
   "postCreateCommand": "setup-dev-env",
+  "postStartCommand": "printenv > .vscode/devcontainer.env",
   "runArgs": [
     "--init"
   ],
   "workspaceFolder": "/root/go/src/github.com/dokku/dokku",
   "workspaceMount": "type=bind,source=${localWorkspaceFolder},target=/root/go/src/github.com/dokku/dokku,consistency=cached"
-}
+}