ci/cd: build and push zot container images

to github's container registry, closes #188

.github/workflows/publish-docker-image.yml

@@ -0,0 +1,24 @@
+name: Publish Docker image
+on:
+  release:
+    types: [published]
+jobs:
+  push_to_registry:
+    name: Push Docker image to GitHub Packages
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out the repo
+        uses: actions/checkout@v2
+      - name: Log in to GitHub Docker Registry
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Build container image
+        uses: docker/build-push-action@v2
+        with:
+          push: true
+          tags: |
+            ghcr.io/${{ github.repository }}:latest
+            ghcr.io/${{ github.repository }}:${{ github.event.release.tag_name }}