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

.env.example

@@ -0,0 +1,7 @@
+# Copy to .env and fill in. Get a fresh registration token from:
+# Gitea -> Site Admin / Org / Repo -> Actions -> Runners -> Create Runner
+GITEA_INSTANCE_URL=https://git.vysma.dev
+GITEA_RUNNER_REGISTRATION_TOKEN=
+GITEA_RUNNER_NAME=changeme-worker
+
+# NOTE: parallel-job count (capacity) is set in config.yaml -> runner.capacity