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}"
}
}