Change Docker image tag versioning style

Author: flaksp

.github/workflows/build.yml

@@ -35,7 +35,7 @@ jobs:
         .
 
     - name: Tag Docker image
-      run: docker tag base ${DOCKER_IMAGE}:${GITHUB_SHA}
+      run: docker tag base ${DOCKER_IMAGE}:build-${{ github.run_number }}
       env:
         DOCKER_IMAGE: docker.pkg.github.com/${{ github.repository }}/observed
 
@@ -44,6 +44,12 @@ jobs:
       env:
         DOCKER_IMAGE: docker.pkg.github.com/${{ github.repository }}/observed
 
+    - name: Remove old Docker images
+      uses: actions/delete-package-versions@v1
+      with:
+        package-name: observed
+        num-old-versions-to-delete: 1
+
   github_pages:
     name: GitHub Pages
     runs-on: ubuntu-latest