Add ability to push/pull docker images to/from Amazon ECR Public Gallery

[devashish-patel]

Description

• This PR adds the ability to push docker images to ECR Public
• From the user's perspective, nothing has been changed. All they need to do is to use the ECR Public URI as the login_server.

Sample Template for review

• Make sure to have right aws credentials set

  
  source "docker" "ubuntu-bionic" {
  image  = "ubuntu:bionic"
  commit = true
  }

build { name = "docker-push-ecr-public" sources = [ "source.docker.ubuntu-bionic" ]

post-processors { post-processor "docker-tag" { repository = "public.ecr.aws/j9y7g6y8/dev_hc_pkr_dkr_test_1" tags = ["packer-rocks", "burrito-boi"] }

post-processor "docker-push" {
  ecr_login    = true
  aws_profile  = "default"
  login_server = "https://public.ecr.aws/j9y7g6y8/dev_hc_pkr_dkr_test_1"
}

} }

Resolves: #45