This is exactly what I do. My product consists of 4 base docker containers. 1 is a nginx reverse proxy to 2 of the 3 remaining containers. 1 is a database container. The 2 other containers hold the API which is "monolithic" app. The other container is just bundled html/css/javascript.
This allows me to scale the API to handle load independent of the app itself, since that's just static assets, it doesn't need it as much.