Add Docker Compose support for one-command installation

[logicguy1]

What are the changes about?

• Adds a complete Docker Compose setup (Dockerfile, docker-compose.yml, docker-entrypoint.sh) so the system can be run with a single docker compose up --build command
• Services included: PHP 8.3 + Apache (web), PostgreSQL 16 (database), Mailpit (email catcher), Adminer (database browser)
• All outgoing emails are caught by Mailpit by default; users can configure a real SMTP server via a .env file without rebuilding the image
• Fixes PHP 8 undefined array key warnings in index/install.php by using the safe if_isset($array, $default, $key) calling convention
• Adds INSTALLATION.md with two collapsible sections, one preserving the original standard installation instructions verbatim, one covering the Docker Compose setup

What's included

File	Purpose
Dockerfile	PHP 8.3 + Apache with pgsql, pdftk, ghostscript, msmtp
docker-compose.yml	Orchestrates web, postgres, mailpit, adminer
docker-entrypoint.sh	Sets permissions, installs Composer deps, generates msmtp config from env vars
.env.example	SMTP configuration template for users
INSTALLATION.md	Updated installation guide with standard + Docker sections

Have you checked the following?

• I have updated relevant documentation at https://github.com/DANOSOFT/saldi/wiki
• I have checked that i am not linking to any external resources such as external style- or javascript-files, that could compomise stabilaty
• I have read and understood the developer guidelines
  https://docs.google.com/document/d/1GOmomtvKf21OV2VWNOIPweDi4gJHpMCK5qXzrPrypUc/edit?usp=sharing