Support --env-file flag for docker compose usage

[adamlewisgucci]

Currently the devcontainer.json spec does not allow for me to specify an .env file for usage in the CLI:
https://docs.docker.com/compose/environment-variables/set-environment-variables/#substitute-with---env-file

This would be really useful, as I currently cannot rely on a root .env file as suggested by the docs, due to collisions with other devs/IDEs and a desire to not pollute each other's ways of working, yet would still like to make use of env var substitution features inside of common .yml files whenever I am running them through the devcontainer stack

[samruddhikhandale]

Similar to microsoft/vscode-remote-release#4885

[nsbradford]

+1 would be very helpful - the docs mention it's supported by runArgs, but doesn't seem to be all connected. There's also --secret-args, but that doesn't seem to work?

https://code.visualstudio.com/remote/advancedcontainers/environment-variables