create

.github/workflows/test.yml

@@ -0,0 +1,33 @@
+# A simple test to test the functionality the action.
+
+name: Test
+on:
+  pull_request:
+  workflow_dispatch:
+  push:
+    branches: [master]
+
+jobs:
+  test-always-true:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      - name: Run
+        uses: ./
+        with:
+          base-image: docker://rust:slim-buster
+          derived-image: docker://rust:slim-buster
+
+  test-always-false:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      - name: Run
+        uses: ./
+        with:
+          base-image: docker://ghcr.io/clementtsang/cargo-deb-arm
+          derived-image: docker://rust:slim-buster

README.md

@@ -1,5 +1,26 @@
 # docker-check-base-image-diff
 
-A Github Action that checks for Docker base image updates. Leverages skopeo and jq.
+A Github Action that checks whether a derived image does not consist of a base image. Leverages
+[`skopeo`](https://github.com/containers/skopeo) and [`jq`](https://stedolan.github.io/jq/).
 
 Based on [lucacome's docker-image-update-checker](https://github.com/lucacome/docker-image-update-checker).
+
+## Usage
+
+Note that this action _assumes_ that `skopeo` is installed; this appears to be true for GitHub's Ubuntu runners.
+
+### Inputs
+
+#### `base-image`
+
+A required argument, this represents the "base" image you want to check against.
+
+As this is based on `skopeo`, it expects a string representing a container, and following a certain format
+(e.g. `docker://rust:slim-buster`). For more information, see [the `skopeo` README](https://github.com/containers/skopeo#skopeo-).
+
+### Outputs
+
+A required argument, this represents the "derived" image you want to check.
+
+As this is based on `skopeo`, it expects a string representing a container, and following a certain format
+(e.g. `docker://rust:slim-buster`). For more information, see [the `skopeo` README](https://github.com/containers/skopeo#skopeo-).

action.yml

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+name: "Docker Check Base Image Diff"
+description: "A Github Action that checks whether a derived image does not consist of a base image."
+author: Clement Tsang <ClementTsang>
+
+inputs:
+  base-image:
+    description: "Base Docker image - follows skopeo format (e.g. docker://rust:slim-buster)"
+    required: true
+  derived-image:
+    description: "Derived Docker image - follows skopeo format (e.g. docker://rust:slim-buster)"
+    required: true
+outputs:
+  differs:
+    description: "True or false"
+    value: ${{ steps.check.outputs.result }}
+runs:
+  using: "composite"
+  steps:
+    - id: check
+      shell: bash
+      run: |
+        result=$(${{ github.action_path }}/check.sh ${{ inputs.base-image }} ${{ inputs.derived-image }})
+        echo "::set-output name=result::${result}"

check.sh

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+#!/bin/bash
+
+BASE_LAYERS="$(skopeo inspect $1 | jq '.Layers')"
+DERIVED_LAYERS="$(skopeo inspect $2 | jq '.Layers')"
+
+jq '.base - .derived | .!=[]' <<< "{\"base\": $BASE_LAYERS, \"derived\": $DERIVED_LAYERS}"