If you are a developer whose only job is to add features to applications that are already on production, then yes Docker is enough (and probably encouraged). Bu

If you are a developer whose only job is to add features to applications that are already on production, then yes Docker is enough (and probably encouraged). But if you are a solo founder, I imagine you're also in charge of infrastructure stuff (provisioning machines, setup backups, monitoring of systems, keep machines updated, etc.): so, for this scenario Docker is not enough; if you want to test your changes locally before applying them in production you must be able to reproduce your production environment locally and that means you need a way to run VMs locally (e.g., via Vagrant + Virtualbox); unless you are willing to have VMs for testing purposes on the cloud (but the same story: Docker is not enough).