Based on the documentation and limitations of DigitalOcean’s App Platform, it seems that running a MongoDB instance in a container within App Platform may not be viable due to several constraints. App Platform doesn’t support persistent storage or VPC networking, which can lead to connectivity and data loss issues when hosting databases directly in containers.

Instead, I recommend using DigitalOcean's Managed Databases service for MongoDB. This managed option provides the necessary persistent storage and network configurations, making it a more suitable choice for database hosting than App Platform, especially for applications needing reliable data storage and availability.

https://docs.digitalocean.com/products/app-platform/details/limits/