ci: add workflow to build InitContainer image

Adds a GitHub Actions workflow to verify the InitContainer Dockerfile
builds successfully.

Signed-off-by: Vanessa Valderrama <vvalderrama@linuxfoundation.org>

Author: vvalderrv

.github/workflows/build-initcontainer.yaml

@@ -0,0 +1,22 @@
+---
+name: Build InitContainer Image
+
+"on":
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build-init:
+    name: Build InitContainer Dockerfile
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v3
+
+      - name: Build InitContainer image
+        run: docker build base/jenkins/init -t initcontainer-test