Move PipeWire Server into a Headless Session

See merge request !19

This is done to move the socket to the directory where the headless pipewire session would expect it.

This is not needed with the headless pipewire setup (A).

This changes the pipewire_socket volume to a bind volume, mapping it to /opt/aura/config/services/pipewire. With this the host can connect to this socket by setting `export PIPEWIRE_RUNTIME_DIR=/opt/aura/config/services/pipewire` and load the client config file with `export PIPEWIRE_CONFIG_FILE=config/pipewire/client.conf`. For this to fully function the host user needs to be able to read and write into `/opt/aura/config/services/pipewire`.

This tests the setup where pipewire fully runs inside docker, #80

Due to the enforcing of unique ids in newer liquidsoap versions, the namespace for auto generated server functions does not comply with the namespace we expect in engine. Here I tested some different approaches in order to try to resolve the issue.

Change `aura` UID:GID to value in the sysuser range

See merge request !18

Collection of fixes found during dev installation

See merge request !15