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Proposal: referencing Docker images consistently

This is a proposal for how to reference Docker images such that the same image is retrieved all the time, regardless of when the request is made.

This will facilitate:
	- scaling up (create new containers using the same image that is currently deployed)

	- rollbacks (create a new container using the exact image that was deployed as part of deployment n-1).

	- Docker stores images in image repositories
	- An image repository is a collection of images and 1 or more tags
	- A tag is a named reference to a specific image ID
	- Images can only be accessed via an image repository
	- Images can only be retrieved by specifying a tag (or, if you don't specify a tag, you'll receive all tagged images)

	- Whenever a tag is created/updated, create an additional tag whose key is the image's ID

	- Example
		- myrepo/myimage:latest => image abcd1234
		- myrepo/myimage:abcd1234 => image abcd1234

	- This allows a client to effectively pull by image ID

	- A Docker Hub image repository that is configured an as automated build (http://docs.docker.com/docker-hub/builds/) does not support manually-created tags, meaning this type of image repository will not work with the above proposal

Hopefully in the future we will be able to reference Docker images by ID, but until that time, we're proposing to use the above mechanism. Please let us know any comments/questions - thanks!


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