What if your application is not necessarily architected for containers? Have you heard of existing systems being successfully migrated without significant changes in software architecture?
Hmm, if it's a monolith, it should be dockerizable without too much trouble. Yes I've migrated several legacy ish systems. Sometimes you have to take shortcuts and be "un-dockerish", for example by running nginx alongside your app server in the same image, but that's not a big deal really in practice.