docs(deploy): add container runner page

website/src/content/docs/deploy/container-runner.mdx

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+---
+title: "Container Runner"
+description: "Run any containerized server as a Rivet Actor."
+skill: true
+---
+
+The container runner (`rivet-container-runner`) turns any container into a Rivet Actor host: as the entrypoint, it spawns your server as a child process and proxies HTTP and WebSocket traffic from the Rivet gateway to it. It is the recommended way to run non-RivetKit software behind Rivet, such as a Unity or Godot dedicated game server, with each container hosting exactly one actor from cold start to exit.
+
+## Install in Your Container
+
+Download the static binary from Rivet's release artifacts in your Dockerfile and set it as the entrypoint, passing your server's launch command after `--`:
+
+```dockerfile @nocheck
+FROM debian:bookworm-slim
+RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates curl && rm -rf /var/lib/apt/lists/*
+
+# Install the Rivet container runner.
+RUN curl -fsSL https://releases.rivet.dev/rivet/latest/container-runner/rivet-container-runner-x86_64-unknown-linux-musl \
+        -o /usr/local/bin/rivet-container-runner \
+    && chmod +x /usr/local/bin/rivet-container-runner
+
+# Your server binary and assets.
+COPY build/ /game/
+WORKDIR /game
+
+ENTRYPOINT ["rivet-container-runner", "--", "./GameServer", "-batchmode", "-nographics", "-logFile", "-"]
+```
+
+Artifacts are published for `x86_64-unknown-linux-musl` and `aarch64-unknown-linux-musl`. The binaries are fully static, so they run in any Linux base image, including `scratch`. Pin a version by replacing `latest` with a release version, for example `https://releases.rivet.dev/rivet/2.3.3/container-runner/rivet-container-runner-x86_64-unknown-linux-musl`.
+
+## How It Works
+
+1. The engine cold-starts your container and calls `POST /api/rivet/start` on the port it injects as `RIVET_PORT`.
+2. The runner spawns your server as a child process with `PORT` set to the child port, waits for the port to open, and reports the actor as running.
+3. Gateway traffic for the actor arrives over Rivet's tunnel and is proxied to `127.0.0.1:<child port>`. WebSocket clients connect through the gateway URL with the `rivet` WebSocket subprotocol.
+4. Child stdout and stderr are re-emitted with an `[actorId=... key=...]` prefix so actor logs are attributed in the dashboard.
+5. When the actor stops, the runner sends the child `SIGTERM`, escalates to `SIGKILL` after a grace period, and exits.
+
+## Configuration
+
+All flags can also be set through environment variables:
+
+| Flag | Environment variable | Default | Description |
+| --- | --- | --- | --- |
+| `--port` | `RIVET_PORT` / `PORT` | `8080` | Serverless front-door HTTP port. Rivet Compute injects `RIVET_PORT` automatically. |
+| `--child-port` | `CHILD_PORT` | `7770` | Local port your server listens on. Exported to the child as `PORT`. |
+| `--actor-name` | `RIVET_ACTOR_NAME` | `game` | Actor name this runner serves. |
+| `--runner-version` | `RIVET_RUNNER_VERSION` | `1` | Version reported to the engine, used to drain old runners on deploy. |
+| `--base-path` | `RIVET_SERVERLESS_BASE_PATH` | `/api/rivet` | Base path the engine calls for serverless start. |
+| `--stop-grace-secs` | `RIVET_STOP_GRACE_SECS` | `25` | `SIGTERM` to `SIGKILL` grace period when stopping the child. |
+| `--readiness-timeout-secs` | `RIVET_READINESS_TIMEOUT_SECS` | `30` | How long to wait for the child's port to open before failing the start. |
+
+### Per-Actor Input
+
+The actor's `input` payload can override the launch spec per actor. All fields are optional and fall back to the entrypoint command:
+
+```json
+{
+	"command": ["./GameServer", "-batchmode"],
+	"args": ["-extra-flag"],
+	"env": { "MATCH_MODE": "ranked" },
+	"port": 7777
+}
+```
+
+`command` replaces the entrypoint command template, `args` are appended to it, `env` adds environment variables for the child, and `port` overrides the child port.
+
+## Deploy
+
+Deploy the image to [Rivet Compute](/docs/deploy/rivet-compute) with the CLI. Because each container hosts one actor, configure the pool with one actor per instance:
+
+```bash
+npx @rivetkit/cli deploy \
+	--token "$RIVET_CLOUD_TOKEN" \
+	--instance-request-concurrency 1 \
+	--dockerfile Dockerfile
+```
+
+Then create actors against the pool's runner (`default`) and connect clients through the gateway URL shown in the dashboard.
+
+## Source and Examples
+
+The runner and a full end-to-end example, including a Unity FishNet demo project and a local test harness, live in the Rivet repository under [`container-runner/`](https://github.com/rivet-dev/rivet/tree/main/container-runner).