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The solution was to mount a helm chart config object in the right place with

--volume $(PWD)/k3d/traefik-config.yaml:/var/lib/rancher/k3s/server/manifests/traefik-config.yaml@all

and the config :

apiVersion: <http://helm.cattle.io/v1|helm.cattle.io/v1>
kind: HelmChartConfig
metadata:
  name: traefik
  namespace: kube-system
spec:
  valuesContent: |-
    additionalArguments:
      - "--<http://entrypoints.web.http.redirections.entryPoint.to|entrypoints.web.http.redirections.entryPoint.to>=websecure"
      - "--entrypoints.web.http.redirections.entryPoint.scheme=https"
      - "--entrypoints.postgres.address=:5432/tcp"
      - "--entrypoints.ssh.address=:22/tcp"
    ports:
      postgres:
        port: 5432
        expose: true
        exposedPort: 5432
        protocol: TCP
      ssh:
        port: 22
        expose: true
        protocol: TCP
        exposedPort: 22