Considering they've been tweeting about it [1] since before their competitors announced things I'd say it's unlikely to be a "response". It's far more likely that Docker has now been out long enough for the various providers to build services around it. AWS already had some docker support built in in April [2]. It's also pretty common to release services as previews. GCE lists theirs as an Alpha quality product.
[1] https://twitter.com/jeffbarr/status/529493907839533056
[2] http://blog.docker.com/2014/04/aws-elastic-beanstalk-launche...