Add public test Docker image build workflow

Builds and pushes ghcr.io/wabolabs/wabosign:test on every master push.
Publicly pullable, no auth required. Non-default tag distinguishes from
stable wabolabs/wabosign releases.

.github/workflows/docker-test.yml

@@ -0,0 +1,41 @@
+name: Build Test Docker Image
+
+on:
+  push:
+    branches:
+      - master
+  workflow_dispatch:
+
+jobs:
+  build-test:
+    runs-on: ubuntu-24.04-arm
+    timeout-minutes: 30
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+      with:
+        submodules: recursive
+
+    - name: Set up QEMU
+      uses: docker/setup-qemu-action@v3
+
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+
+    - name: Login to GitHub Container Registry
+      uses: docker/login-action@v6
+      with:
+        registry: ghcr.io
+        username: ${{ github.actor }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: Build and push test image
+      uses: docker/build-push-action@v6
+      with:
+        context: .
+        push: true
+        platforms: linux/amd64,linux/arm64
+        tags: ghcr.io/${{ github.repository }}:test
+        cache-from: type=gha
+        cache-to: type=gha,mode=max