feat: build oCIS from git source and add CI smoke test

[DeepDiver1975]

Summary

• Replace pre-built binary download with a full source build from the oCIS git repository (two new builder stages: node-builder for IDP/web frontend assets, go-builder for the CGO binary)
• Drop v7/7.3.2 (dead Kopano upstream dependency prevents IDP build)
• Enable BuildKit GHA cache for faster CI builds
• Add server smoke test: starts the container, polls https://localhost:9200/status.php for HTTP 200, and verifies .productversion matches the built tag

Test Plan

• CI build completes successfully for v8
• Smoke test passes: server starts, HTTP 200 on /status.php, .productversion matches 8.0.1

🤖 Generated with Claude Code

[DeepDiver1975]

Added BuildKit GHA cache in the latest commit.

This depends on owncloud-docker/ubuntu#266 landing first (adds the docker-cache-from/docker-cache-to inputs to the reusable workflow). Once that merges to master, the cache will be active — warm re-runs of the same version tag should drop from ~78 min to a few minutes.