Add new flexible parameters and new logic of using of nominatim-docker

[altair86]

When I made these changes, my goal was:

to make nominatim deployment and update secure.

To do this, I added:

• New parameter SKIP_IMPORT, which allows skipping imports when we have a "naive" container that is intended only for updating
• I also had to add PGDATABASE to the documentation and change a few lines in which the nominatim database participates in order to avoid erroneous database deletion on production systems
• Based on this, I added new work cases called "Using an external PostgreSQL database and configuring with updating the data container"

Thank you for your work

[what-the-diff[bot]]

• Added PGDATABASE variable to init.sh and start.sh
• Changed default value of PGDATABASE from nominatim to $PGDATABASE in init.sh
• Fixed typo in line 49 (init script) - !-z -> ! -z