[Fix] Any version set to TFLINT_TAG, TERRAFORM_DOCS_TAG, TFENV_VERSION must exist in the DOCKER_REGISTRY

[arongate]

Tools (TFEnv, TFLint, Terraform-docs) are built and published to the registry configured at DOCKER_REGISTRY. So if you configure a version which has not been built and published in the DOCKER_REGISTRY yet you can't use the tools version in your project.
It forces us to request a new release any time we want to use a version of the tools which does not already exists in the registry at DOCKER_REGISTRY.

Proposed Solution
Make using a central registry optional and not mandatory to workaround this issue.