Gitea Act Runner: DinD stack with cache + parallel jobs

Stable docker-compose setup for building Rust projects on Gitea Actions.
DinD-isolated daemon, built-in cache server, capacity=4 parallel jobs,
healthcheck-gated startup. Portable to other build machines via .env.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HU4wDwdydNZpR3YkMiYYy8

.gitignore

@@ -0,0 +1,8 @@
+# Secrets — never commit the real registration token
+.env
+
+# Runner local data (if ever bind-mounted)
+data/
+
+# Tooling / session state
+.omc/