Migrate workflow "systemtest-base-el8" to Github Actions

This commit adds a new GitHub Actions workflow to build the Vespa
systemtest base Docker image for CentOS 8. The workflow is triggered
on pushes to the main branch and pull requests targeting the main
branch, and it builds and pushes the Docker image to Docker Hub.
 image.
feat(ci): Schedule container image build in weekly cron

.github/workflows/build-container-images.yml

@@ -0,0 +1,59 @@
+name: Build Container Images
+
+on:
+  push:
+    paths:
+      - ".github/workflows/build-container-images.yml"
+      - "docker/**"
+
+  pull_request:
+    branches:
+      - main
+    paths:
+      - ".github/workflows/build-container-images.yml"
+      - "docker/**"
+
+  schedule:
+    - cron: "0 4 * * 1"
+
+jobs:
+  systemtest-base-el8:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Docker metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: docker.io/vespaengine/vespa-systemtest-base-el8:latest
+          tags: |
+            # tag container image with branch name and pr number
+            type=ref,event=branch
+            type=ref,event=pr
+            # set "latest" tag for default branch
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push
+        uses: docker/build-push-action@v
+        with:
+          context: docker/
+          file: Dockerfile.base
+          build-args: |
+            BASE_IMAGE=docker.io/almalinux:8
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}