Support dotenv files

[kindermax]

Currently the only way to run lets with predefined specific env variables is to use sh, for example:

Env script .env.sh

export MY_ENV=123

lets.yaml

In this case MY_ENV exists.

commands:
  run: echo ${MY_ENV}

But the problem that if I want to explicitly tell lets command which env vars I expect, there is only one way using sh

This does not work - it prints ${MY_ENV}

commands:
  env:
    MY_ENV: ${MY_ENV}
  run: echo ${MY_ENV}

This work - it prints 123

commands:
  env:
    MY_ENV: 
      sh: echo ${MY_ENV}
  run: echo ${MY_ENV}

We have two options:

1. Add env interpolation such as in docker-compose so this will print 123, but the problem is that this requires distinguish between local and interpolated vars. Docker compose uses $$ to access local vars. I do not want to add this right now.

commands:
  env:
    MY_ENV: ${MY_ENV}
  run: echo ${MY_ENV}

2. Add support for dotenv. This can be implemented in two ways:

First: here we are declaring env and env_file - so env can interpolate vars from file

commands:
  env:
    MY_ENV: ${MY_ENV}
  env_file: .env
  run: echo ${MY_ENV}

Second: or we can just populate env from env_file. This I like most - because it does not requires us to implement interpolation right now, it is just appends env from file to existing env.

Questions:

1. What priority env_file has ?
2. Do we need env_file global keyword ?

commands:
  env_file: .env
  run: echo ${MY_ENV}