ci: add github workflow for publishing container images

Publishes container images of the reference plugins at
ghcr.io/containerd/nri/plugins/<plugin>:<tag>

The tag is the git tag name for tagged versions and "unstable" for the
main branch.

Author: marquiz

.github/workflows/images.yml

@@ -0,0 +1,46 @@
+name: Publish Container Images
+
+on:
+  push:
+    branches:
+      - main
+    tags:
+      - v[0-9]+.[0-9]+.[0-9]+
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  publish-images:
+    runs-on: ubuntu-latest
+    permissions:
+      packages: write
+      contents: read
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Log in to registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push images
+        env:
+          IMAGE_REPO_BASE: ghcr.io/${{ github.repository }}/plugins
+        run: |
+          case ${{ github.ref }} in
+              refs/heads/main)
+                  export VERSION=unstable
+                  ;;
+          esac
+
+          make plugin-images
+          make plugin-images-push
+