ci: add GitHub Actions workflow for UBI9 Docker image builds

[ncarenton]

Proposed changes

Port the UBI9 Docker image build pipeline introduced in OpenCTI-Platform/connectors#6542 to this repository.

• Add .github/ubi9-injectors.json listing all injectors eligible for UBI9 image builds, used to generate the build matrix dynamically
• Add .github/workflows/build-ubi9.yml triggered on push to main, release/** branches and version tags ([0-9]*.[0-9]*.[0-9]*); runs the existing tests as a prerequisite job, then builds and pushes multi-platform (linux/amd64,linux/arm64) UBI9-based Docker images for each listed injector
• Modify .github/workflows/test-injectors.yml to support workflow_call so it can be reused as a prerequisite job inside the build pipeline

Notes

• All GitHub Actions must be pinned by SHA for supply-chain security
• GHCR support can be commented out initially; only DockerHub active for image pushes
• Image tag is computed from the git ref: version tag → X.Y.Z-ubi9, main → latest-ubi9, branch → <sanitized-branch>-ubi9
• Reference implementation: OpenCTI-Platform/connectors#6542