Manage system time zone

[davpsh]

Description

This PR contains changes proposed in #887 (comment)

Changes

• adds an explicit default system time zone UTC
• adds system time zone output in debug endpoint (/debug)
• adds TIMEZONE building argument to build image with prefered time zone

Examples

Build image with prefered time zone

docker build --build-arg TIMEZONE=America/New_York ... 

Run image with prefered time zone

docker run --rm -p "3000:3000" gotenberg/gotenberg:8 gotenberg -e TZ=America/New_York

todo

• add examples in documentation

[davpsh]

Important! To correctly install and configure the time zone, the tzdata package must be installed.
In some common Linux distributions, such as Debian and CentOS, the tzdata package is already installed in the base Docker images. If the base image changes, we will need to add the commands to install the package.

[gulien]

Could you add a comment about that? So that the futur me does not miss it 😄