Support standard GRANIAN_* env vars and the rloop event loop

- Launcher now only injects --interface/--host/--port when the matching
  GRANIAN_* env var is unset, so every granian option (GRANIAN_LOOP,
  GRANIAN_WORKERS, GRANIAN_HTTP, GRANIAN_HOST, GRANIAN_PORT, ...) can be
  configured via granian's standard GRANIAN_* environment variables
- Add the rloop event loop as a runtime dependency so GRANIAN_LOOP=rloop
  works out of the box (previously failed with a missing 'rloop' package)

README.md

@@ -52,6 +52,35 @@ Configuration is read from environment variables:
 | `RBCS_DIGEST`           | *(unset)*     | Hash the key with this algorithm (e.g. `sha256`) |
 | `RBCS_MAX_AGE`          | `7d`          | Value TTL (`s`/`m`/`h`/`d`)         |
 
+### Configuring Granian
+
+The server runs on [Granian](https://github.com/emmett-framework/granian) via
+its RSGI interface. Any granian option can be configured with its standard
+`GRANIAN_*` environment variables (see `granian --help`), for example:
+
+```bash
+GRANIAN_LOOP=rloop GRANIAN_WORKERS=4 GRANIAN_HTTP=2 rbcs-server
+```
+
+Every granian CLI option has a matching `GRANIAN_*` variable
+(`GRANIAN_LOOP`, `GRANIAN_WORKERS`, `GRANIAN_HTTP`, `GRANIAN_TASK_IMPL`,
+`GRANIAN_RUNTIME_THREADS`, `GRANIAN_BLOCKING_THREADS`, `GRANIAN_LOG_LEVEL`,
+`GRANIAN_METRICS`, `GRANIAN_WS`, ...).
+
+When a `GRANIAN_*` variable and the corresponding `RBCS_*` variable are both
+set, the granian one wins:
+
+- `GRANIAN_HOST` overrides `RBCS_HOST` (default `127.0.0.1`)
+- `GRANIAN_PORT` overrides `RBCS_PORT` (default `8080`)
+- `GRANIAN_INTERFACE` overrides the default `rsgi` interface
+
+The Docker image ships the `rloop` event loop, so `GRANIAN_LOOP=rloop` works
+out of the box:
+
+```bash
+docker run -e GRANIAN_LOOP=rloop -e RBCS_MEMCACHE_HOST=memcached kaya-rbcs
+```
+
 The app is also a plain ASGI application, so it can be served by any ASGI
 server, e.g. `daphne kaya_rbcs.app:app`.