Describe the bug
using docker compose, mounted volume for /etc/timezone is ignored

To Reproduce

volumes:
  - '/etc/timezone:/etc/timezone:ro'

Expected behavior
Expect to see watchtower operate in mounted timezone in notification timestamps and schedule checking.

Environment

• ubuntu 18.04 LTS
• x86
• Docker version 20.10.5, build 55c4c88
• containrrr/watchtower latest c992dfdeb81b 4 days ago 16.3MB
• timezone is "America/Chicago" in my case

Additional context
watchtower operates in correct timezone if environmental variable TZ is set.