feat: allow changing Docker image for scan job

[krzysztof-clari]

Description:

Allows changing Docker image used for the scan job, this is useful if the Docker images should be pulled from a different place than the GitHub Container Registry. Such situations usually occur with air-gapped systems or places where Docker registry access is restricted.

Fixes #4241.

Checklist:

• [x] Tests passing (make test-community)?
• [ ] Lint passing (make lint this requires golangci-lint)?
  • Not really applicable as the change does not touch Go code, but no, fails with:

❯ make lint
golangci-lint run --enable bodyclose --enable copyloopvar --enable misspell --out-format=colored-line-number --timeout 10m
Error: unknown flag: --out-format
Failed executing command with error: unknown flag: --out-format
make: *** [lint] Error 3

My version information:

❯ golangci-lint --version
golangci-lint has version 2.1.6 built with go1.24.2 from eabc263 on 2025-05-04T15:36:41Z