Validate docker image before bumping

[joemaller]

Check that the image exists before bumping. v5.9.2 currently does not exist on DockerHub, but it's the latest release returned from the WordPress API.

There are some notes here:

docker-wordpress-dev/docker-compose.yml

Lines 41 to 47 in 1eea447

	# TODO: Check DockerHub for existance of the latest release beffore continuing?
	# https://registry.hub.docker.com/v2/repositories/library/wordpress/tags/5.9.1
	# unknown tags return a 404:
	# https://registry.hub.docker.com/v2/repositories/library/wordpress/tags/7.9.4

Might not be able to catch the 404, but checking for a name might be enough?

Here's the DockerHub error JSON for a non-existent image:

{
  "errinfo": {
    "namespace": "library",
    "repository": "wordpress",
    "tag": "5.9.5"
  },
  "message": "tag '5.9.5' not found"
}

A successful request is much bigger, but includes a top-level name key.