Fix bbb_exporter docker deployment

[Chasethechicken]

Mount /etc/bigbluebutton/bigbluebutton-release in bbb_exporter docker container.
This is needed for bbb_exporter to export the deployed BBB version.

Otherwise bbb_exporter:latest fails to start with:

bbb-exporter   | Traceback (most recent call last):
bbb-exporter   |   File "server.py", line 46, in <module>
bbb-exporter   |     REGISTRY.register(collector)
bbb-exporter   |   File "/usr/local/lib/python3.8/site-packages/prometheus_client/registry.py", line 24, in register
bbb-exporter   |     names = self._get_names(collector)
bbb-exporter   |   File "/usr/local/lib/python3.8/site-packages/prometheus_client/registry.py", line 64, in _get_names
bbb-exporter   |     for metric in desc_func():
bbb-exporter   |   File "/app/collector.py", line 111, in collect
bbb-exporter   |     with open("/etc/bigbluebutton/bigbluebutton-release", "r") as f:
bbb-exporter   | FileNotFoundError: [Errno 2] No such file or directory: '/etc/bigbluebutton/bigbluebutton-release'
bbb-exporter exited with code 1