So, I managed to get it working. Just coming here to post what works for me if anyone have the same issue. I try to scape double quotes, stringify, etc.
I had to "unfold (?)" the admin object and then all start working:
{ "url": "${URL}", "port": "${PORT}", "admin":{ "username": "${ADMIN_USERNAME}", "email": "${ADMIN_EMAIL}", "password": "${ADMIN_PASSWORD}", "password:confirm": "${ADMIN_PASSWORD}" }, "database": "${DB_VENDOR}", "${DB_VENDOR}": { "host": "${DB_HOST}", "port": "${DB_PORT}", "username": "${DB_USER}", "password": "${DB_PASSWORD}", "database": "${DB_NAME}", "ssl": "${DB_SSL}" } }
