Support passing env vars from a file

[drehelis]

A feature request to have an option to specify --env-file argument (multiple?) with the mirrord binary and/or env: {"env_file": file} in the configuration file.

Use-case:
We manage a long list of env vars in bunch of .env files and use packages such as python-dotenv, godotenv and npms's dotenv etc. to load them.

Currently, the only option is to explicitly type them on the command line / source it or duplicate it to a mirrord.json config file and use -f switch

Current situation:

ENV=production DB_HOST_NAME=postgres.host DB_USER_NAME=xxx DB_PASSWORD=yyy DB_NAME=zzz ./mirrord exec npx -- sequelize-cli db:migrate

Expected:

./mirrord exec --env-file ./env -- npx sequelize-cli db:migrate

Discussion on Discord,