fix(ci): Add DOCKER_HOST environment variable for ElasticsearchControllerSpec

ElasticsearchControllerSpec was failing in CI due to Docker connection issues.
Added DOCKER_HOST=unix:///var/run/docker.sock to properly configure Docker client
connection in GitHub Actions Ubuntu environment.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: j5ik2o

.github/workflows/ci.yml

@@ -51,6 +51,7 @@ jobs:
       JVM_OPTS:  -Xms2048M -Xmx2048M -Xss6M -XX:ReservedCodeCacheSize=256M -Dfile.encoding=UTF-8
       AWS_REGION: ap-northeast-1
       TEST_TIME_FACTOR: 5
+      DOCKER_HOST: unix:///var/run/docker.sock
     steps:
       - uses: actions/checkout@v5.0.0
         with: