No error for --env-file if the ./env file does not exist

[asglover]

Description

This would be really nice QoL improvement for devcontainers. Many devs will have personal .env files with API keys for services, but it's annoying to have to make an empty .env file just so that the docker CLI doesn't error at you. You can do a workaround by using "initialize command" "source .env". I believe this behavior is different than docker compose which lets you provide a path with no file actual file necessary.

I believe the fix would be straightforward just having a different default behavior for the parser which powers this features. If you would like I can make a PR.