Add docker login to dind for internal registry access

The host Docker daemon is authenticated via the runner's job-started
hook, but the dind service is a separate daemon with no auth config.
Log in to docker.internal.networkradius.com via dind before pulling.

Secrets are passed via env vars to avoid shell interpolation.

Author: arr2036

.github/workflows/ci-multi-server-tests.yml

@@ -204,6 +204,17 @@ jobs:
         with:
           lfs: false
 
+      #  Authenticate to the internal registry via the dind daemon.
+      #  The host Docker daemon is logged in via the runner's
+      #  job-started hook, but dind is a separate daemon with no
+      #  auth config.
+      - name: Login to internal Docker registry
+        env:
+          DOCKER_USERNAME: ${{ secrets.DOCKER_REPO_USERNAME }}
+          DOCKER_PASSWORD: ${{ secrets.DOCKER_REPO_PASSWORD }}
+        run: |
+          echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin https://docker.internal.networkradius.com/
+
       - name: Get pre-built Docker image for self-hosted runner test
         shell: bash
         env: