Add GetContainerPlatforms a useful reusable workflow to get

list of supported platforms (architectures) as well as
corresponding json usable for matrix.

Author: sobomax

.github/workflows/GetContainerPlatforms.yml

@@ -0,0 +1,27 @@
+name: Get Container Platforms
+description: 'Extracts supported platforms from a Docker image manifest'
+inputs:
+  image:
+    description: 'Image name to inspect'
+    required: true
+outputs:
+  platforms:
+    description: 'Comma-separated list of platforms'
+    value: ${{ steps.set-env.outputs.platforms }}
+  build-matrix:
+    description: 'JSON array of platforms'
+    value: ${{ steps.set-env.outputs.build-matrix }}
+
+runs:
+  using: 'composite'
+  steps:
+  - name: Set dynamic environment
+    id: set-env
+    run: |
+      PLATFORMS="$(docker manifest inspect ${{ inputs.image }} | \
+        jq -r '.manifests[] | "\(.platform.os)/\(.platform.architecture)\(if .platform.variant != null then "/\(.platform.variant)" else "" end)"' | \
+        sort -u | grep -v unknown | paste -sd ',')"
+      BUILD_MATRIX="$(echo ${PLATFORMS} | tr ',' '\n' | jq -R . | jq -s . | tr '\n' ' ')"
+      echo "platforms=${PLATFORMS}" >> $GITHUB_OUTPUT
+      echo "build-matrix=${BUILD_MATRIX}" >> $GITHUB_OUTPUT
+    shell: bash