Set CONTEXT when running Netlify Dev

[jon-sully]

- Summary

In the last few months the addition of Build Plugins (particularly https://github.com/bencao/netlify-plugin-inline-functions-env) combined with the deploy context env var have made it possible for Functions to feel environment-agnostic a la:

const contextualEnvVar = (v) => {
  const currentContext = process.env.CONTEXT
  const formattedContext = currentContext.replace('-', '_').toUpperCase()
  return process.env[`${formattedContext}_${v}`]
}

exports.handler = async _ => {
  return {
    statusCode: 200,
    body: JSON.stringify({
      status: contextualEnvVar('API_KEY')
    })
  }
}

I'll be writing up a post on this soon, but the premise is that I can set PRODUCTION_API_KEY, DEPLOY_PREVIEW_API_KEY, and DEV_API_KEY all in the Netlify Admin UI then my function will correctly get the key value for each environment using contextualEnvVar('API_KEY') as shown above.

The ultimate result is that my Function will run with the correct ENV vars for each environment it's in, I can (securely) set ENV var values via the Netlify Admin UI for all four environments (dev, deploy-preview, branch-preview, and production), and I can easily override the dev key locally by just adding the key to my local .env file (adding DEV_API_KEY in this case).

This is really neat. Using Stripe as the canonical example, this structure would let me have my Stripe Prod and Testing keys configured safely in the Netlify Admin UI and feel good knowing that my local dev, deploy-previews, and production environments would be using the correct keys all the time.

This PR just adds CONTEXT = dev to the process thread when running netlify dev locally. Currently no CONTEXT is set and I think this just adds parity to Netlify's other environments where CONTEXT is set.

- Description for the changelog

Add CONTEXT=dev to environment variables when running netlify dev

- What else

I currently have this working great on a micro testing site, Tenv (test env)! Here are a couple of cURLs to show:

$ curl https://deploy-preview-2--tenv.netlify.app/.netlify/functions/output-env-var

$ curl https://tenv.netlify.app/.netlify/functions/output-env-var

The site can be inspected as needed too. tenv.netlify.app

I believe this addition / workflow would also satisfy a few different community requests.

- A picture of a cute animal (not mandatory but encouraged)

[jon-sully]

I'm not sure why some of the CI builds failed 😕 A couple of the Windows builds on the first pass and now one of the ubuntu builds

[erezrokah]

Thanks @jon-sully, the CI failure seems unrelated. This is also related to #473 (comment).
Please bare with us while we discuss this internally to figure out what are the best built in env variables the CLI should inject.

[erezrokah]

Thanks @jon-sully this is good to go and sorry for the delay in approving it.

[jon-sully]

Yay! Thanks!