Instructions for overriding body with file content

Author: mendhak

README.md

@@ -33,6 +33,7 @@ This image is executed as non root by default and is fully compliant with Kubern
 - [Configuring CORS policy](#setting-corscross-origin-resource-sharing-headers-in-the-response) 
 - [Client certificate details (mTLS) in the response](#client-certificate-details-mtls-in-the-response)
 - [Preserve the case of headers in response body](#preserve-the-case-of-headers-in-response-body)
+- [Override the response body with a file](#override-the-response-body-with-a-file)
 - [Prometheus Metrics](#prometheus-metrics)
 - [Screenshots](#screenshots)
 - [Building](#building)
@@ -287,6 +288,16 @@ By default, the headers in the response body are lowercased. To attempt to prese
 docker run -e PRESERVE_HEADER_CASE=true -p 8080:8080 -p 8443:8443 --rm -t mendhak/http-https-echo:33
 ```
 
+## Override the response body with a file
+
+To override the response body with a file, set the environment variable `OVERRIDE_RESPONSE_BODY_FILE_PATH` to a file path.  
+The file path needs to be in the `/app` directory.  
+
+```bash
+docker run -d --rm -v ${PWD}/test.html:/app/test.html -p 8080:8080 -e OVERRIDE_RESPONSE_BODY_FILE_PATH=/test.html -t mendhak/http-https-echo:33
+```
+
+
 ## Prometheus Metrics
 
 To expose http performance metrics, set the `PROMETHEUS_ENABLED` environment variable to true, the metrics will be available at `/metrics`. This uses the [`express-prom-bundle`](https://github.com/jochen-schweizer/express-prom-bundle) middleware