Expand environment variables when using useDotenv: true in serverless configuration

[d-asensio]

Use case description

Currently, when enabling the useDotenv flag, the .env file that matches the specified stage (.evn.local, .env.production, etc...) using dotenv directly. However dotenv do not expand environment variables, so users of serverless can't combine them in their .env files.

As a serverless user, I would find it very useful to be able to combine environment variables in such way:

API_BASE_URL=my.api.com
API_PROTOCOL=https

API_URL=$API_PROTOCOL/$API_BASE_URL

Proposed solution

As it is solved in other packages (as dotenv-cli) and as it is recommended by the README of dotenv I would suggest using dotenv-expand for this purpose.

I have created a PR with the proposed changes above.