Comment 2 for bug 1806504

Try using cloudinit-userdata model-config:

cat ./modelconfig.yaml
cloudinit-userdata: |
  ca-certs:
    trusted:
    - |
      -----BEGIN CERTIFICATE-----
      MIIFgDCCA2igAwIBAgIJANG/4O762QdKMA0GCSqGSIb3DQEBCwUAMFUxCzAJBgNV
      # ... rest of the cert here
      -----END CERTIFICATE-----
  preruncmd: |
    - echo this will run before
    - echo juju runcmd
  postruncmd: |
    - echo this will run after
    - echo juju runcmd

juju model-config modelconfig.yaml