feat: Add support for additional docker_image build contexts

[Multox]

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Description

Docker now has the ability to use multiple build contexts. Add support for this feature.
Either change build.context to an array or add build.additional_contexts.

New or Affected Resource(s)

• docker_image

Potential Terraform Configuration

resource "docker_image" "zoo" {
  name = "zoo"
  build {
    context = [".", "../someotherdir"]
    #OR (backwards-compatible version)
    additional_contexts = ["../foo", "../bar"]
  }
}

References

[https://www.docker.com/blog/dockerfiles-now-support-multiple-build-contexts]