Should worker processes of web applications be placed in separate containers?
I can think of only one case where overhead of running workers in separate containers is justified: if your setup uses docker swarm for clustered deployment. This way you will get all of the HA benefits.

Otherwise I don't see a reason to complicate such tasks, especially if they must use strictly the same codebase (which leads me to believe that they share one domain of concerns, so to say). It might seem elegant, but I just don't see enough benefits

p.s. any split of such kind will force you to manage interconnection and complicate setup for local development