docker port_mapping option doesn't seem to work right...

If you have the following in mx-tester.yml

```
docker:
hostname: synapse
port_mapping:
- host: 8081
guest: 8008
```

Then the host port seems to be ignored and mx-tester still uses `9999`.

Start by reproducing the configuration from mx-tester.yml with host port 8081 and guest port 8008, then trace how the docker port_mapping option is read and why 9999 is still used. Done means the configured host port is honored instead of the default, with a regression check for this configuration.

Written by the indexing model from the issue text.