chore: add example for audit logging to file in docker

examples/audit/README.md

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+# Audit Event Logging Example
+
+**This feature is still under development, and this example is subject to change.**
+
+This example shows how you can run Flipt with Audit Event logging enabled to a file on disk.
+
+This works by setting the two environment variables `FLIPT_AUDIT_SINKS_LOG_ENABLED` and `FLIPT_AUDIT_SINKS_LOG_FILE`:
+
+```bash
+FLIPT_AUDIT_SINKS_LOG_ENABLED=true
+FLIPT_AUDIT_SINKS_LOG_FILE=/var/opt/flipt/audit.log
+```
+
+The auditable events currently are CRUD (except for read) operations on `flags`, `variants`, `segments`, `constraints`, `rules`, `distributions`, and `namespaces`. If you do any of these operations through the API, it should emit an audit event log to the specified location.
+
+Since docker containers are ephemeral and data within the container is lost when the container exits. In this example we mount a local file on the host to the audit event log location in the container as a volume. You would have to create the file [first](https://github.com/moby/moby/issues/21612#issuecomment-202984678) before starting the container:
+
+```bash
+mkdir -p /tmp/flipt && touch /tmp/flipt/audit.log
+```
+
+and `tail` the logs as you are making API request to the Flipt server when the container is running.
+
+```bash
+tail -f /tmp/flipt/audit.log
+```
+
+## Requirements
+
+To run this example application you'll need:
+
+* [Docker](https://docs.docker.com/install/)
+* [docker-compose](https://docs.docker.com/compose/install/)
+
+## Running the Example
+
+1. Run `docker-compose up` from this directory
+1. Open the Flipt UI (default: [http://localhost:8080](http://localhost:8080))

examples/audit/docker-compose.yml

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+version: "3"
+
+services:
+  flipt:
+    build: ../../
+    command: ["./flipt", "--force-migrate"]
+    volumes:
+      - /tmp/flipt/audit.log:/var/log/audit.log
+    ports:
+      - "8080:8080"
+    environment:
+      - FLIPT_LOG_LEVEL=debug
+      - FLIPT_AUDIT_SINKS_LOG_ENABLED=true
+      - FLIPT_AUDIT_SINKS_LOG_FILE=/var/log/audit.log