[bradstewart]

Docker isn't just a packager--it adds a whole nother layer of runtime to your app.

Access to network/disk/other IO, logging, init systems, configuration, etc are all different under Docker. There's more pieces of software which can crash (docker engine, etc). Not to mention potential performance issues and/or other subtle differences which may occasionally occur.

This isn't to say Docker has no value--it does, and we use it in production on a number of applications. But it is certainly more than just a package.