Update docker/login-action from v2 to v3 (#398)

v2 uses Node.js 16 which is deprecated and will stop working June 2026.

Co-authored-by: Nick Satterly <nsa@softpay.io>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

.github/workflows/docker.yml

@@ -27,7 +27,7 @@ jobs:
           -t $REPOSITORY_URL/$IMAGE_NAME:$(git rev-parse --short HEAD)
           -t $REPOSITORY_URL/$IMAGE_NAME:latest .
       - name: Docker Login
-        uses: docker/login-action@v2
+        uses: docker/login-action@v3
         with:
           registry: ${{ env.REPOSITORY_URL }}
           username: ${{ github.actor }}