Add ability to use toml file instead of .env

[deny-7]

Description

It would be great to add the ability to use a toml file instead of a regular .env, because toml provides much more functionality than a regular .env.

e.g.

services:
  web:
    image: ${toml.services.images}
    ports: ${toml.services.ports}
    env_file: 
       - ${toml.services.env}
  redis:
    image: ${toml.services.images}

TOML example

[services]

[services.web]
image = "nginx"
ports = { 80 = 80, 443 = 443 }
env = {somevar1 = someval1, somevar2 = someval2}

[services.redis]
image = "redis"

[ndeloof]

That's an interesting proposal, some thoughts:

• Compose would need to know about the current service node to resolve toml.services.images, this is a significant change in the parser
• Your example assumes variable can be resolved into an array (see ports) which is also a major challenge (see Allow an environment variable to be used for an array field (e.g. devices:) #4249) we haven't addressed yet