Version 4.3.6: MQTT documentation and Docker build improvements and a bug fix

This update contains an MQTT documentation update, Docker build updates and a minor bug fix. It is recommended for all users.

Bug Fix

• Fix a potential bug (no issues reported) by checking that a buffer is not zero before attempting to deallocate it.

Documentation Updates

• Show how to use MQTTThing with Homebridge to represent Shairport Sync’s active status. Thanks to keefar for the PR.
• Correct and clarify the statistics documentation.
  1. The descriptions of Net Sync PPM and All Sync PPM were partly swapped -- this has been corrected.
  2. Generally clarify the language and harmonize the wording with the introduction.
     Many thanks to willmo for the PR.
• Fix a typo in the configuration file. Many thanks to Justin.

Docker Build Optimisation

Improve the Dockerfile build sequences as follows:

1. Build each dependency in a separate stage, facilitating parallel builds.
2. Define default value for ARGS, permitting easier local docker builds.
3. Clone the specified branch only, and to a depth of 1, thus getting relevant data faster.
4. Copy files in a one-off command, reducing the amount of Docker layers created.

Many thanks to David Girón for the PR.