Support declaring environment variables in DVC file

[robogeek]

Obviously some command-line tool configuration is set using environment variables. DVC does not support setting environment variables in a DVC file. Neither DVC file reference nor dvc run command reference show anything about setting an environment variable.

As to how it should be visible to the users:

• dvc run -e VARIABLE=value -- support setting this option multiple times
• In the top level of a DVC file have an env tag, like so:

cmd: echo This is a command
env:
    - VARIABLE_NAME=value
    - VARIABLE_NAME2=value
    - VARIABLE_NAME3=value

OR

cmd: echo This is a command
env:
    VARIABLE_NAME: value
    VARIABLE_NAME2: value
    VARIABLE_NAME3: value

For comparison, this is exactly how environment variables are defined in a Docker Compose file: https://docs.docker.com/compose/compose-file/#environment

[efiop]

Great idea @robogeek ! We've actually been discussing something like this (more like 2nd option, which is closer to travis/appveyor yaml syntax) with @MrOutis and @pared . Along with dvc being able to resolve things like $(VARIABLE_NAME) in commands and paths, it should get us covered for the parametrization feature as well. 🙂

[shcheklein]

might be a duplicate of #1416 to some extent, there was a discussion about introducing -e

[efiop]

@shcheklein Indeed, closing in favor of that.

[shcheklein]

@robogeek please, give a vote to that initial ticket. And thanks for the feedback :)