fix(scanner): correct startup user defaults and API scan identity

[pedropombeiro]

Summary

Fix startup user handling and restore Docker builds on master:

• Correct the scanner-name fallback, default empty UID/GID values, and reject invalid IDs.
• Write the resolved scanner UID to config.php so web scans use the account created at startup.
• Use Bash parameter expansion for subnet parsing to satisfy ShellCheck.
• Move to Bookworm to resolve the expired Bullseye metadata failure.
• Replace the unavailable Brother download with the brscan-skey and brscan-skey-libsane1 packages already used on development.
• Document ID defaults. The commit hook also formatted the README.

Validation

• Full Docker build and PR build-and-publish workflow passed.
• Seven isolated container cases passed against the resulting image, covering default, custom, zero, empty, and invalid settings.
• Web scan dispatch ran a test script under the expected UID/GID. Driver package installation, lighttpd configuration, and commit hooks passed.

Physical scanning and driver registration remain unverified.