New feature: selection of a custom binary

[GioF71]

We can not select a custom binary using the variables:

• SELECT_CUSTOM_BINARY_ALSA
• SELECT_CUSTOM_BINARY_PULSE

for ALSA and PULSE respectively.
Binaries shoud be placed in new volume /app/assets/binaries preferably.

Example:

---
version: '3.3'

services:
  squeezelite-d10-4x-nodsd:
    image: giof71/squeezelite:latest
    container_name: squeezelite-d10-4x-nodsd
    restart: unless-stopped
    devices:
      - /dev/snd:/dev/snd
    environment:
      - PRESET=topping-d10-sw,archimago-goldilocks,rates_up_to_192k
      - SQUEEZELITE_MODEL_NAME=topping-d10-4x-nodsd
      - SQUEEZELITE_EXCLUDE_CODECS=dsd
      - SQUEEZELITE_NAME=kodi-living-pi4-d10-4x-nodsd
      - SQUEEZELITE_SERVER_PORT=lms.homelab.local
      - SELECT_CUSTOM_BINARY_ALSA=/app/assets/binaries/alsa/squeezelite
    volumes:
      - ./bin:/app/assets/binaries:ro

The binary file must be executable.