Add support to use system environment variable for tox-docker container

[Girish Sharma]

Is there any way with which I can pass my system's or tox's environment variable (set via setenv or passenv) to tox-docker container?

I want my container to have a environment variable but don't want to track that in git.

[Terry Tu Nguyen]

You can achieve it through this @letmerecall

[docker:my_service]
image = my-service-image:latest
environment =
    VAR1=aaaaa
    VAR2={env:VAR2}