provider/docker: authentication via values instead of files

[mkuzmin]

This PR adds new parameters for configuring remote Docker host for encryption and client authentication via inline values instead of external files.

provider "docker" {
  host = "tcp://hostname:2376"
  ca_material = "${tls_self_signed_cert.docker_ca.cert_pem}"
  cert_material = "${tls_locally_signed_cert.docker_client.cert_pem}"
  key_material = "${tls_private_key.docker_client.private_key_pem}"
}

This allows referencing other resources, for example from TLS and Vault providers.

See a complete example at https://github.com/mkuzmin/terraform-demo/tree/a851f57feda0e694e6a908baff019d6ea79b0316

[stack72]

Hi @mkuzmin

Thanks so much for the PR here - so, we have been moving towards doing something like key_material rather than paths to files. I just merged the work for moving the chef provider towards this for 0.8 (#10105)

If possible, could you follow the same pattern here?

Thanks

Paul

[mkuzmin]

I've renamed all three parameters, and also add handling for environment variables.
Please take a look.

[stack72]

Looks good to me now :) Thanks for following up so fast on this :)

P.

[ghost]

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