Add sample for OCI context with base image trigger

Signed-off-by: Sajay Antony <[email protected]>

Author: sajayantony

windows-oci-context/README.md

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+# Using an OCI context
+
+The sample below builds a windows image based on an OCI context.
+
+> These are `powershell` commands. 
+
+## Bundle up the context into an OCI Artifact
+
+The context has a simple `Dockerfile` and a `hello.txt`
+Use oras to create the context and ensure you are in `context` directory. 
+
+```shell
+$REGISTRY="myregistry"
+$REGISTRY_LOGIN_SERVER="myregistry.azurecr.io"
+cd context
+az acr login -n $REGISTRY
+oras push $REGISTRY_LOGIN_SERVER/windows/oci-context:latest .
+```
+
+## Create an ACR to use the context
+
+Create a base image trigger enabled task with the new context and ensure that you specify the required `--platform`. 
+
+```powershell
+$REGISTRY="myregistry"
+$REGISTRY_LOGIN_SERVER="myregistry.azurecr.io"
+az acr task create `
+  --registry $REGISTRY `
+  --name ocicontextbuild `
+  --platform Windows/amd64 `
+  --image "windowsimage:{{.Run.ID}}" `
+  --context "oci://$REGISTRY_LOGIN_SERVER/windows/oci-context:latest" `
+  --file Dockerfile `
+  --commit-trigger-enabled false `
+  --base-image-trigger-enabled true
+```
+
+## Run the task to build and register the base image
+
+Execute the task to build the container image. 
+
+```powershell
+$REGISTRY="myregistry"
+az acr task run --name ocicontextbuild --registry $REGISTRY
+```
+
+The output should confirm that the base images were identified. 
+
+```
+...
+- image:
+    registry: myregistry.azurecr.io
+    repository: windowsimage
+    tag: cf1bun
+    digest: sha256:90f3c6534f9ed4ed8fa5bdaca006e284f0900bb19a850c0a249bede83bb6ed70
+  runtime-dependency:
+    registry: mcr.microsoft.com
+    repository: windows/servercore
+    tag: ltsc2019
+    digest: sha256:2bacc4bdc5d1bd805587cb90a2fb2d58d1c775b02df1a19fd221cbfc639ff587
+  git: {}
+
+
+Run ID: cf1bun was successful after 5m1s
+```

windows-oci-context/context/Dockerfile

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+# escape=`
+FROM mcr.microsoft.com/windows/servercore:ltsc2019
+
+# Copy a simple text file into the image
+COPY hello.txt C:\hello.txt
+
+# When the container runs, display the contents of hello.txt
+CMD ["cmd", "/c", "type C:\\hello.txt && timeout /t 10"]
+

windows-oci-context/context/hello.txt

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+Hello from Windows container!