fix(ci): expose Docker socket env vars for Testcontainers in backend job

DOCKER_HOST makes the socket explicit rather than relying on runner
config propagation; TESTCONTAINERS_RYUK_DISABLED=true avoids Ryuk
watchdog start failures in nested container environments.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -80,6 +80,8 @@ jobs:
     runs-on: ubuntu-latest
     env:
       DOCKER_API_VERSION: "1.43"  # NAS runner runs Docker 24.x (max API 1.43); Testcontainers 2.x defaults to 1.44
+      DOCKER_HOST: unix:///var/run/docker.sock
+      TESTCONTAINERS_RYUK_DISABLED: "true"
     steps:
       - uses: actions/checkout@v4