✨ Add dotenv file hierarchy

[wwilsman]

Purpose

As discovered in #199 the Node dotenv package does not support dotenv file hierarchy out-of-the-box like dotenv-rails does. Since test commands are run as a child process of the percy command, the .env file has already been loaded into the environment via dotenv and will not be overridden by dotenv-rails.

Approach

The dotenv-rails readme outlines the order of priority for various supported files. Since dotenv will only set env vars that have not been previously set, we can load the files in priority order to mimic dotenv-rails hierarchy.

In addition to properly setting variables as expected to be set by dotenv-rails, this also makes it so various PERCY_* environment variables can be set in environment specific dotenv files as well.

The NODE_ENV environment variable must be set for environment specific dotenv files. For example, NODE_ENV=production will load .env.production.local, .env.local, .env.production, and .env files in that order.

Questions

As mentioned in #199, we might be wary of continuously adding support for different dotenv language behaviors. I personally like dotenv-rails approach and think it's fine to add this support so we can capture PERCY_* variables defined in those files. As for other languages, what are their approaches (if any) and should we include a way to disable loading .env files completely in this library?

[Robdel12]

and should we include a way to disable loading .env files completely in this library?

Probably. I don't feel like it's a high priority, but I think it's the right thing to also offer.

[Robdel12]

🏁 Good stuff, I dig it. We should document this somewhere too