Set default thread count based on nproc

[philipkozeny]

fixes https://github.com/mediagis/nominatim-docker/issues/458

sets the default amount of threads to the cpus available via nproc while still allowing the override with the existing THREADS environment variable.

[what-the-diff[bot]]

PR Summary

• Elimination of THREADS Environment Variable in Dockerfiles In this change, the THREADS environment variable is removed from the Dockerfiles of version 4.2 and 4.3. Previously, you had to manage the setting of this variable manually, but that is no longer necessary.

• Update of THREADS Variable Description in README Files The README files for version 4.2 and 4.3 have been updated. They now state that the THREADS variable's default value will be the number of processing units that the program can use. The program automatically determines this, freeing you from having to do it manually.

• Removal of Restart Property in Docker Composition File The restart property has been removed from the Docker composition file for version 4.2. This change simplifies the configuration of the file.

• Automated Setup of THREADS Variable in Initiation Scripts The initiation scripts for versions 4.2 and 4.3 have a new automatic setup for the THREADS variable. If you haven't set up the variable or if it's empty, the script will make it equal to the number of process units the program can utilize. This automation makes the setting up of the program less cumbersome.