I’m using the nano editor thru the terminal to make some changes. However for attaching file here I’ve connected to my Droplet by using Cyberduck and copied the body of the YAML file by using a usual text editor on Mac 
Does nano editor and text editor convert the tab gaps to the space gaps? I’ve just saved that file on the text editor and upload to the containers folder, but didn’t help
Also, I’m wondering, why the YAML checker didn’t show tab spaces for you, when you checked the app.yml file, if there it could happen?