Pass environment variables from cog.yaml to build stage

[okapusta]

Allow to pass environment variables to docker file template during build

Example:

build:
  gpu: false
  environment:
    - ENV_VAR_NAME=some_value
    - SOME_OTHER_VAR=anything_goes
predict: predict.py:Predictor

[mcam10]

This would be good to have... IMO..

[AlexanderKozhevin]

wassup

[bfirsh]

Thank you for the contribution! This was added in #2274 at the top level, then #2807 moves it into the build: section.