Read env vars and substitutions from file

[GordianDziwis]

My workaround for env vars is to put export $(cat .env | xargs) into my scripts and substitutions I read with:

IFS="="; while read -r key value; do echo -n " -s _$$key=$$value"; done < .env | xargs -o popper run -f wf.yml

It would be nice if popper would support reading from an .env file directly.