All of the Heroku button stuff is configured in an `app.json` file in the root of the repository. There's also still a Procfile used. The `app.json` just helps with the initial configuration such as environment variables that need to be set, scripts to run, etc.
I'm not sure how much you know about Heroku, but their runtime environment is based around so-called buildpacks, which provide the required executables, packages, configuration, etc. for various languages (ruby, php, js, ...). Once you push code to Heroku's git remote it will analyze your code and select the proper buildpack to run the code on.
So you are absolutely right that you can't simply go from an app.json file to a local development system in one step, as you (very likely) don't have the required infrastructure on your system.
I see - so the idea behind checking in a Procfile is that you can run this locally using "heroku local", but the idea behind a app.json is that you explicitly can't?
The Procfile describing your application given some base installation (e.g. PHP 7.3), the app.json describing in this case nothing in particular?
Any application requires both a base environment and instructions for application deployment. And that is how the Dockerfile for rss-bridge is constructed, too.
So what besides vendor lock-in is the advantage of Heroku's approach?
Heroku's buildpacks make some default assumptions regarding an app's startup. For a Ruby on Rails app, for example, it would simply start the web server using `bundle exec rails s` unless you define something else in the Procfile. I'd assume there is a similar procedure for PHP apps, probably starting an nginx instance and pointing it at the app's index.php or something like that.
The way I understood it, app.json is there to customize the environment for the application to run in – providing ENV variables, required addons (think databases, memcache, etc.) and pre/post deploy scripts. A sort of configuration-as-code if you will. It is explicitely not used to define the actual processes that should be started when the app is deployed, that's what the Procfile is for, as long as you're running in an environment that supports Procfiles.
I'm not really sure what to tell you regarding vendor lock-in. Apart from the app.json the repo itself looks completely vendor-unaware, as it is simply a PHP application. It doesn't seem to make many assumptions regarding your (local) infrastructure but rather assumes you know how to get a PHP application to run on your server/computer. The presence of the app.json file is just an affordance to those who would want to try out the app without having to configure anything themselves.
On the contrary, now that I think of it. I always found Heroku to be rather non-locking, as you can just take your code and run it somewhere else. You need to provide some additional tooling around your deployments yourself in those cases, but that's true for all PaaS providers, isn't it? Heroku Addons are nice features, but usually simply services provided by third parties that are made available using automatically generated ENV variables, which you could simply copy over to wherever else your app is running.
