fix: permadiff issue when docker_repository field is not specified

[kumadee]

When docker repository is not specified, by default docker repository projects/PROJECT_ID/locations/REGION/repositories/gcf-artifacts is used. Refer https://cloud.google.com/functions/docs/reference/rest/v2beta/projects.locations.functions#dockerregistry

In such a case, terraform plan always generates a difference for docker_repository field.

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # module.foo.google_cloudfunctions2_function.function will be updated in-place
  ~ resource "google_cloudfunctions2_function" "function" {
        id               = "projects/my-project-e044dd03/locations/europe-west1/functions/foo"
        name             = "foo"
        # (10 unchanged attributes hidden)

      ~ build_config {
          - docker_repository     = "projects/my-project-e044dd03/locations/europe-west1/repositories/gcf-artifacts" -> null
            # (4 unchanged attributes hidden)

            # (1 unchanged block hidden)
        }

        # (2 unchanged blocks hidden)
    }

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

This fix prevents unnecessary infrastructure change by setting the default repository id received from API.

Relates to GoogleCloudPlatform/cloud-foundation-fabric#1987

Release Note Template for Downstream PRs (will be copied)

Fixed an issue when `docker_repository` is not specified in build_config section of the `google_cloudfunctions2_function`, the provider should not generate a diff during plan for the default value used by the API.

[modular-magician]

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 1 file changed, 1 insertion(+))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+))

[modular-magician]

Tests analytics

Total tests: 14
Passed tests 13
Skipped tests: 1
Affected tests: 0

Click here to see the affected service packages

• cloudfunctions2

$\textcolor{green}{\textsf{All tests passed in REPLAYING mode.}}$
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