Docker.github.io: Compose file reference doc references `.env` file support

[dnephin]

See https://github.com/moby/moby/issues/29133#issuecomment-309429376

https://docs.docker.com/compose/compose-file/#variable-substitution

We should either remove the reference to .env from this document or clarify that it's only a docker-compose feature.

[bastiaanb]

@dnephin : or create feature parity by adding a parameter to docker stack create to specify an env file?
I understand docker stack to be a replacement to docker-compose, but it is not?
Thanks!

[dnephin]

Even if we were to add .env file support I don't think it's correct to document it here. This file should be a reference for the format. At most we should link to another page.

I understand docker stack to be a replacement to docker-compose, but it is not?

Maybe eventually, but certainly not now. They support the same format (with a few exceptions) but the commands are designed for different use cases. docker-compose is designed for development and testing workflows, docker stack deploy is designed for more general use cases (including production). The trade offs are a bit different. One is not a drop-in replacement for the other.

[thaJeztah]

ping @londoncalling

[fedaykin]

@dnephin I believe this is where docker-compose.yml syntax is documented, both for Compose (docker-compose) and for docker stack deploy. Throughout this page is documentation specifying which features and what syntax is available via either docker-compose or docker stack deploy. For the '.env' feature, one is simply led to conclude that it's available for both, and I'm of the opinion it should be.

[dperetti]

New users first learn about docker-compose.yml (the file) when they experiment with docker-compose (the command).
Then they get into to swarm stuff, and it's still referred to as a docker-compose.yml file.
The documentation should maybe use a different name, like docker stack deploy -c docker-swarm.yml.

[londoncalling]

@dperetti this is a good idea to differentiate and clarify the Compose filename for new users. However, It will be a bit of work because there are a lot of moving parts (asciinema, screen snaps, example code, and possibly more ...)

Let's see if we can get @johndmulhausen, @mistyhacks, or @JimGalasyn to weigh in on it for bandwidth right now. What do you guys think?