Improve logging

- Give means to increase logging verbosity
- Explain how to do in the troubleshooting guide

docs/install/docker.md

@@ -20,3 +20,6 @@ docker run --rm -e MATRIX_DOMAIN=example.org -v /data/mxisd/etc:/etc/mxisd -v /d
 ```
 
 For more info, including the list of possible tags, see [the public repository](https://hub.docker.com/r/kamax/mxisd/)
+
+## Troubleshoot
+Please read the [Troubleshooting guide](../troubleshooting.md).

docs/troubleshooting.md

@@ -19,6 +19,11 @@ If you use the [Docker image](install/docker.md), this goes to the container log
 
 For any other platform, please refer to your package maintainer.
 
+### Increase verbosity
+To increase log verbosity and better track issues, the following means are available:
+- Add the `-v` command line parameter
+- Use the environment variable and value `MXISD_LOG_LEVEL=debug`
+
 ### Reading them
 Before reporting an issue, it is important to produce clean and complete logs so they can be understood.