Restore pulse:test image build in the e2e workflow

The Core E2E workflow on this branch lost the 'docker build -t
pulse:test --target runtime .' line that release/5.1 still has, so
every dispatched run failed at compose-up with 'pull access denied for
pulse'. Masked until now because the workflow only auto-triggers on
main.

.github/workflows/test-e2e.yml

@@ -52,6 +52,7 @@ jobs:
 
       - name: Build Docker images for test environment
         run: |
+          docker build -t pulse:test --target runtime .
           docker build -t pulse-mock-github:test ./tests/integration/mock-github-server
         env:
           PULSE_LICENSE_PUBLIC_KEY: ${{ secrets.PULSE_LICENSE_PUBLIC_KEY }}