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Container in docker see each other with the container-names as domainname and the internal ports. Request-URL ‘php-backend:80’ should do the trick.
Edit: Just realized the ajax-request is from your local browser, not from within the todo-app-container, the answer would not help. But then I do not see the problem. The request to ‘localhost:8080’ should go to the php-backend
Source:stackexchange.com