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 (depth), getting data faster.
+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 GirĂ³n](https://github.com/duhow) for the [PR](https://github.com/mikebrady/shairport-sync/pull/1945).