fix: use 0.0.0.0 as webserver default address in sample config

When using 127.0.0.1 as default, docker containers won't work at all without mounting a custom config file into the container, because the application listens to localhost. The Docker build copies the sample config into the image. This way the application works just fine without specifying a custom config if you're happy with using defaults.

config.sample.yaml

@@ -1,5 +1,5 @@
 webserver:
-  listen_addr: "127.0.0.1"
+  listen_addr: "0.0.0.0"
   listen_port: 8080
   full_url: "/full-stream"
   lite_url: "/"