docs: suggest mounting localtime, not timezone

[piksel]

The current docs suggests mounting the hosts /etc/timezone into the watchtower container, but that has no effect.

/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 in watchtower, and since the image is a base image, no service that updates that symlink ever runs.

Instead, this would change the docs to suggest directly mounting the hosts /etc/localtime file.

Fixes #876

[codecov]

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