There is a need for 2 Kube secrets:

loomio:
  DATABASE_URL: postgresql://postgres:passwordgoeshere@loomio-db/loomio_mayara
  DEVISE_SECRET: 'uniquehash'
  SECRET_COOKIE_TOKEN: 'uniquehash'
loomio-db:
  POSTGRES_PASSWORD: passwordgoeshere

Those secrets are read as environment variables as per the deployment template

There is a flag to enable db initialisation in values.yaml. It must be set to true, helm install then set it to false and helm upgrade