feat: Add FastAPI s6 sidecar PoC

- Sidecar DragonflyDB as Redis replacement
- Run pytheus in multiprocessing mode with Redis backend

README.md

@@ -0,0 +1,13 @@
+# FastAPI - s6-overlay - sidecar PoC
+
+### How-To:
+
+    docker build -t fastapi-s6-sidecar-poc
+    docker run --rm -p 8080:80 fastapi-s6-sidecar-poc
+    curl http://localhost:8080/metrics
+
+### Sources:
+- [s6-overlay for docker](https://github.com/just-containers/s6-overlay)
+- [FastAPI](https://fastapi.tiangolo.com)
+- [Pytheus FastAPI Prometheus Middleware](https://github.com/Llandy3d/pytheus)
+- [DragonflyDB - Redis drop-in replacement](https://github.com/dragonflydb/dragonfly)