Specify docker_repository field for google_cloudfunctions2_function

[kumadee]

When docker repository is not specified, by default docker repository projects/PROJECT_ID/locations/REGION/repositories/gcf-artifacts is used.
In such a case, terraform plan always generates a difference for docker_repository field as the module passes null value but the tfstate file has the above specified default value.

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 allows one to prevent unnecessary infrastructure change by setting the default repository id as well as any user created repository.

---

Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

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[ludoo]

The permadiff should be fixed in the provider, as optional fields which are not specified should not generate a change once the resource is created.

Adding the variable to be able to use a non-default repository does of course make sense, thanks for this PR.

[kumadee]

The permadiff should be fixed in the provider, as optional fields which are not specified should not generate a change once the resource is created.

Adding the variable to be able to use a non-default repository does of course make sense, thanks for this PR.

Cool. I can raise the PR to fix the permadiff in the provider next week.

[ludoo]

Cool. I can raise the PR to fix the permadiff in the provider next week.

this would be great, thanks a lot! :)