add docker_registry_image resource with build support

[edgarpoce]

Summary

Add a new resource called docker_registry_image that provides an image/tag in a docker registry such as AWS ECR

resource "docker_registry_image" "helloworld" {
  name = "1234567890.dkr.ecr.us-east-1.amazonaws.com/helloworld:1.0"
  build {
    context = "pathToContextFolder"
  }
}

Schema proposal

• name - (Required, string) The name of the Docker image.

• keep_remotely - (Optional, boolean) If true, then the Docker image won't be
  deleted on destroy operation. If this is false, it will delete the image from
  the docker registry on destroy operation.

• build - (Optional, Map) ... arguments from docker cli golang library

Implementation proposal

• Optionally build the image if a build configuration is provided
• Create a context folder hash to identify whether the source files changed, store the context folder hash in the tf state

References

• No support for docker build using dockerfiles #33

[mavogel]

closed with #249