Actually I suggest writing less code, assuming that Docker is a whole lot of code and "what if we write down dependencies" is more interface than code.
Your hypothetical vision of the future sounded like you were suggesting writing a tool/system to manage the complexity of dependencies/deployment. I'd recommend changing the thing-to-be-deployed-itself, so it was simpler and had fewer dependencies (if we're talking hypothetical code changes).
Aside: complex software is hard to deploy because it's complex. Software gets complex because devs are very comfortable managing large amounts of complexity. Docker lets you hide that complexity so someone other than the dev can do the build/test/deploy steps, but it does nothing to address the high level of complexity in the underlying software. Let's not mince words here: Discouse is hard to deploy because Rails wasn't designed to make its projects easy to deploy by someone other than the devs primarily working on it, and no amount of tooling around it will change that.
Aside: complex software is hard to deploy because it's complex. Software gets complex because devs are very comfortable managing large amounts of complexity. Docker lets you hide that complexity so someone other than the dev can do the build/test/deploy steps, but it does nothing to address the high level of complexity in the underlying software. Let's not mince words here: Discouse is hard to deploy because Rails wasn't designed to make its projects easy to deploy by someone other than the devs primarily working on it, and no amount of tooling around it will change that.