watchtower ignores mounted /etc/timezone file

[scottjl]

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.

[github-actions]

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[piksel]

I modified the watchtower startup to print some TZ info:

Output from running outside docker in a "clean" multipass VM

ubuntu@primary:~/dev/watchtower$ go run .
Local location: Local
LOC: 2021-04-02 11:48:11.084724485 +0200 CEST m=+1.003943893
UTC: 2021-04-02 09:48:11.084724485 +0000 UTC
/etc/timezone: Europe/Stockholm
Manually loaded location: Europe/Stockholm
ETC: 2021-04-02 11:48:11.084724485 +0200 CEST
/usr/share/zoneinfo/Europe/Stockholm: 1909 byte(s)
Environment:
  SHELL=/bin/bash
  PWD=/home/ubuntu/dev/watchtower
  LOGNAME=ubuntu
  XDG_SESSION_TYPE=tty
  MOTD_SHOWN=pam
  HOME=/home/ubuntu
  LANG=C.UTF-8
  LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=00:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arc=01;31:*.arj=01;31:*.taz=01;31:*.lha=01;31:*.lz4=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.tzo=01;31:*.t7z=01;31:*.zip=01;31:*.z=01;31:*.dz=01;31:*.gz=01;31:*.lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.zst=01;31:*.tzst=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.alz=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.cab=01;31:*.wim=01;31:*.swm=01;31:*.dwm=01;31:*.esd=01;31:*.jpg=01;35:*.jpeg=01;35:*.mjpg=01;35:*.mjpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.m4a=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:*.oga=00;36:*.opus=00;36:*.spx=00;36:*.xspf=00;36:
  SSH_CONNECTION=192.168.112.1 50436 192.168.113.245 22
  LESSCLOSE=/usr/bin/lesspipe %s %s
  XDG_SESSION_CLASS=user
  TERM=xterm-256color
  LESSOPEN=| /usr/bin/lesspipe %s
  USER=ubuntu
  SHLVL=1
  XDG_SESSION_ID=32
  XDG_RUNTIME_DIR=/run/user/1000
  SSH_CLIENT=192.168.112.1 50436 22
  XDG_DATA_DIRS=/usr/local/share:/usr/share:/var/lib/snapd/desktop
  PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
  CGO_ENABLED=0
  DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
  SSH_TTY=/dev/pts/0
  OLDPWD=/home/ubuntu
  _=/usr/bin/go
  DOCKER_HOST=unix:///var/run/docker.sock
  DOCKER_API_VERSION=1.25

Output running inside a docker container:

ubuntu@primary:~/dev/watchtower$ go build && docker build -t wtdev -f dockerfiles/Dockerfile .
ubuntu@primary:~/dev/watchtower$ docker run --rm -it -v /var/run/docker.sock:/var/run/docker.sock -v /etc/timezone:/etc/timezone:ro  wtdev:latest --run-once
Local location: UTC
LOC: 2021-04-02 09:48:56.284432609 +0000 UTC m=+0.003314540
UTC: 2021-04-02 09:48:56.284432609 +0000 UTC
/etc/timezone: Europe/Stockholm
Manually loaded location: Europe/Stockholm
ETC: 2021-04-02 11:48:56.284432609 +0200 CEST
/usr/share/zoneinfo/Europe/Stockholm: 1909 byte(s)
Environment:
  PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
  HOSTNAME=6594f073fc1b
  TERM=xterm
  HOME=/
  DOCKER_HOST=unix:///var/run/docker.sock
  DOCKER_API_VERSION=1.25

Container was built using dockerfiles/Dockerfile and started with a bound /etc/timezone.
Everything looks like it SHOULD work, but for some reason it's not picking up the timezone from /etc.

Manually loading the correct timezone does work, but according to the docs the file it uses for local time is /etc/localtime and not /etc/timezone... So let's try that...

ubuntu@primary:~/dev/watchtower$ docker run --rm -it -v /var/run/docker.sock:/var/run/docker.sock -v /etc/localtime:/etc/localtime:ro  wtdev:latest
Local location: Local
LOC: 2021-04-02 11:55:56.204415857 +0200 CEST m=+1.007427835
UTC: 2021-04-02 09:55:56.204415857 +0000 UTC
INFO[0001] Watchtower v0.0.0-unknown
Using no notifications
Checking all containers (except explicitly disabled with label)
Scheduling first run: 2021-04-03 11:55:56 +0200 CEST

Indeed, that works. We should update the docs to reflect this.

Some more details regarding this:
/etc/localtime is usually a symlink to /usr/share/zoneinfo/<contents of /etc/timezone>, but that symlink has to be created. So whatever is in /etc/timezone does not affect the actual tz database being used, and since the image is a base image, no service that updates that symlink ever runs.

The only potential problem I can see with binding the hosts /etc/localtime are binary incompability between systems, but that doesn't seem like it should be an issue, since the format is system-independent (bigendian, regardless of arch. etc), defined in an RFC and hasn't changed in a long time.

[ghost]

To avoid important communication to get lost in a closed issues no one monitors, I'll go ahead and lock this issue. If you want to continue the discussion, please open a new issue. Thank you! 🙏🏼