Deploy app runner from repo subfolder

I am trying to deploy a NodeJs express app using AWS app runner. The issue is that my source code for both the frontend and backend are contained in the same repo under different subfolders (i.e. my express app is located in 'repo/backend', while my frontend is in 'repo/frontend'). Unfortunately, I haven't been able to get app runner to deploy my express application from the repo folder.

My build command is: npm install My start command is: npm start

Note that I have been able to deploy the app successfully when the backend source code is uploaded in its own dedicated GitHub repo (i.e. from the repo's parent directory). So it does seem that the code being in its own subfolder within the repo is the issue.

Am I missing some configuration here or command? Any ideas would be very much appreciated!