I really don't understand how all these "instead of Heroku just use X"-people do not understand that one of the main benefits of Heroku is not managing servers. If your app on Heroku has an issue Heroku will fix it (not your app of course). If your app on Docker has an issue. Who you gonna call?
Recently, Google App Engine allows deploying apps in any language using Docker. So, with Docker, App Engine is suddenly a competition to Heroku for Ruby programmers.
Yeah, if your app can fit on Heroku, leave it there. But Heroku is not a silver bullet for all types of apps. Containerizing them makes it much easier to add and remove instances, easily deploy new services, move instances around as underlying resources die, etc. Docker doesn't provide everything to make this happen but it's the key.
I've spent the past two weeks moving an Akka cluster into Docker with CoreOS. I'm about to deploy Kafka in the same way and so far it's been fantastic.
But you couldn't before either and yet Heroku still existed, so how does that make Docker an Heroku-killer?
The groundwork is being laid and I think we're going to see a lot of competition in Heroku's space. Given the price difference between Heroku and EC2 (much less DigitalOcean) I think there's a lot of profit margin for competitors to attack. That's Heroku's weakness: it's easier than ever to build the equivalent functionality.
Docker makes these services relatively comparable to Heroku in complexity, so the heroku value prop has been diluted.
Put another way, hardware stores sell paint and paint supplies all day. Buying those and painting a room yourself is certainly cheaper, but it assumes you have the extra time to paint and don't mind you being the laborer. Hiring a painter costs more, but saves you a ton of time, energy, and elevates the level of expertise you are bringing to the job.
Getting started is one thing. The ease of how you maintain and grow is another.
Downtime, on the other hand, is just lost revenue opportunity.
(Heroku Cost) <=> (EC2 Cost) + (EC2 DevOps Cost)
Now we have:
(Heroku Cost) <=> (EC2 Cost) + (EC2 Devops using Docker)
Where (EC2 Devops using Docker) < (EC2 Devops)
So the value proposition has shifted. It's not a "Heroku Killer", but it does dilute their value prop, as my prior post mentioned.
For most well designed services, that is simply not the case. If my revenue from $1000 of heroku bills is $20,000, cutting $500/mo off of my server costs in exchange for any amount of hours or risk is simply not worthwhile.