document copa github action

Signed-off-by: ashnamehrotra <[email protected]>

docs/github-action.md

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+# Copa Github Action
+
+## Overview
+
+The [Copa Github Action](https://github.com/project-copacetic/copa-action) allows you patch vulnerable containers in your workflows using Copa. 
+See Copa Action docs for example usage.
+
+## Inputs
+
+## `image`
+
+**Required** The image reference to patch.
+
+## `image-report`
+
+**Required** The trivy json vulnerability report of the image to patch.
+
+## `patched-tag`
+
+**Required** The new patched image tag.
+
+## `buildkit-version`
+
+**Optional** The buildkit version used in the action, default is latest.
+
+## `copa-version`
+
+**Optional** The Copa version used in the action, default is latest.
+
+## Output
+
+## `patched-image`
+
+Image reference of the resulting patched image.