Fix panic to migrate schema of docker_container from v1 to v2

[suzuki-shunsuke]

Closes #264

=> #264 (comment)

If "mounts" isn't set in Terraform Configuration with Docker provider v2.6.0,
"mounts" isn't set in State.
So rawState doesn't have the key "mounts" and rawState["mounts"].([]interface{}) occurs panic.

[suzuki-shunsuke]

The panic is solved, but I found the other problem.
The container is recreated due to the change of env.

#264 (comment)

$ terraform apply
$ vi main.tf # upgrade docker provider from 2.6.0 to 2.7.0 or 2.7.1
$ terraform init
$ terraform plan
      ...
      env.#:            "" => <computed> (forces new resource)
      ...

$ terraform plan
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.

docker_image.nginx: Refreshing state... (ID: sha256:4392e5dad77dbaf6a573650b0fe1e282b57c5fba6e6cea00a27
c7d4b68539b81nginx)
docker_container.iis_services: Refreshing state... (ID: de1e5024beba5f0c8dd4c1d4bfcc2e45ff23ad61441653f
5a0753120b3536f7f)

------------------------------------------------------------------------

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
-/+ destroy and then create replacement

Terraform will perform the following actions:

-/+ docker_container.iis_services (new resource required)
      id:               "de1e5024beba5f0c8dd4c1d4bfcc2e45ff23ad61441653f5a0753120b3536f7f" => <computed
> (forces new resource)
      attach:           "false" => "false"
      bridge:           "" => <computed>
      command.#:        "3" => <computed>
      container_logs:   "" => <computed>
      entrypoint.#:     "1" => <computed>
      env.#:            "" => <computed> (forces new resource)
      exit_code:        "" => <computed>
      gateway:          "172.17.0.1" => <computed>
      hostname:         "de1e5024beba" => <computed>
      image:            "sha256:4392e5dad77dbaf6a573650b0fe1e282b57c5fba6e6cea00a27c7d4b68539b81" => "s
ha256:4392e5dad77dbaf6a573650b0fe1e282b57c5fba6e6cea00a27c7d4b68539b81"
      ip_address:       "172.17.0.2" => <computed>
      ip_prefix_length: "16" => <computed>
      ipc_mode:         "private" => <computed>
      labels.#:         "0" => <computed>
      log_driver:       "json-file" => <computed>
      log_opts.%:       "0" => <computed>
      logs:             "false" => "false"
      must_run:         "true" => "true"
      name:             "iis-services" => "iis-services"
      network_data.#:   "1" => <computed>
      read_only:        "false" => "false"
      restart:          "always" => "always"
      rm:               "false" => "false"
      shm_size:         "64" => <computed>
      start:            "true" => "true"
      user:             "" => <computed>


Plan: 1 to add, 0 to change, 1 to destroy.

------------------------------------------------------------------------

Note: You didn't specify an "-out" parameter to save this plan, so Terraform
can't guarantee that exactly these actions will be performed if
"terraform apply" is subsequently run.

If we remove the Computed: true from env, this problem is solved.
But I don't know whether we should remove Computed: true from env or not.

https://github.com/terraform-providers/terraform-provider-docker/blob/v2.7.1/docker/resource_docker_container.go#L459

[suzuki-shunsuke]

In my understanding, by #269 we stop to import env to State so we can remove Computed: true from env.

[mavogel]

LGTM. Thanks for the fix. I'll set up a call in the upcoming weeks with the hashicorp folks to determine the correct usage of all the attributes.