Optimized dockerfiles - PHP 8.1, volume and xdebug

[ram-fabian-heinrich]

Based on https://github.com/anuko/timetracker/pull/127:

• Install all application dependencies in the same block

• Add --no-install-recommends switch to reduce image size

• Install all PHP extensions in the same block

• Add version to DB base image to fix Dockerfilelint issue

Additional changes:

• Removed the copyiny of files to /var/www/html. Added a volume instead. This volumes syncs local files with files in docker container.
• Configuration files must now be created manually. Added step to CONTRIBUTING.md.
• Added xdebug extension.
• Docker php version from 7.2 to 8.1.

[anuko]

Hello, it may take me a while to review this due to other emergencies.