Add proposal for container tags

docs/proposals/container-tags.md

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+<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>
+
+If you are using a released version of Kubernetes, you should
+refer to the docs that go with that version.
+
+Documentation for other releases can be found at
+[releases.k8s.io](http://releases.k8s.io).
+</strong>
+--
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+
+# Container Tags
+
+## Abstract
+
+We propose to extend `PodSpec` and `Container` with a way for the
+client to specify metadata tags to appear on the containers of the
+pod.  This supports use cases in which Kubernetes is not the only
+system involved; in such situations there are reasons why the user may
+wish to put some metadata on the containers.  Examples include tagging
+to identify the tenant in a multi-tenant scenario (at least, when a
+tenant can have more than one k8s namespace), and supporting tooling
+and practices that operate on containers.
+
+The multi-tenant scenario has been discussed with the networking SIG,
+and this model change has been briefly discussed in the Node SIG.
+
+## Model changes
+
+There are two model changes.  The `PodSpec` will be extended with a
+field holding a set of tags that is applied to every container of the
+pod.  The `Container` is extended with a field holding a set of tags
+that apply to just that container.
+>NOTE: Initially the model changes will be implemented as annotations.
+
+```go
+// ContainerTags is a set of metadata tags that is applied to
+// every container of the pod.  The exact tagging mechanism is
+// specific to the container runtime being used.  The user should use
+// keys and values that the container runtime can support.
+type map[string]string ContainerTags
+
+// PodSpec is a description of a pod
+type PodSpec struct {
+	...
+
+	// Tags that apply to every container of the pod.
+	ContainersTags ContainerTags `json:"containersTags,omitempty"` 
+}
+
+...
+
+// Container represents a single container that is expected to be run on the host.
+type Container struct {
+	...
+
+	// Tags that apply to this container.
+	ContainerTags ContainerTags `json:"containerTags,omitempty"` 
+}
+```
+
+## Prototyping via annotations
+
+The above extensions will be represented at first as a json encoded annotations like so:
+
+
+```yaml
+kind: Pod
+apiVersion: v1
+metadata:
+  annotations:
+    containers-annotations.alpha.kubernetes.io: |
+      {
+        "foo": "bar",
+        "baz": "oof"
+      }
+    container-annotations.alpha.kubernetes.io: |
+      {
+        "containerX": {
+          "tag11": "val11",
+          "tag12": "val12",
+        },
+        "containerY": {
+          "tag21": "val21"
+        }
+      }
+...
+```
+
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