--set FOO=bar could work. Up to give it a try?

You can also use a valid shell command in a .env file.

--set FOO=bar could work. Up to give it a try?

I get

[Symfony\Component\Console\Exception\RuntimeException]  
  The "--set" option does not exist. 

You mean trying to implement it @nicolas-grekas ?

Yes :)

Just a quick note: I also think the logic should be part of the DotEnv component itself. So that we can call $dotEnv->dumpToFile($path....) or whatever. Right now, you cannot build your own dump-env command without copying loads of stuff from Flex. But you might want to use the component outside of Flex which is why it was designed as a component in the first place, right? :-)

This can be achieved by dumping the extra env vars in eg .env.local if I'm not wrong.
There's also this effort to move that command to Symfony: symfony/symfony#42610
I'm closing therefor.