Use prebuilt image builder with all Java versions.
Download MxBuild inside container (improves layer reuse).

This helps if Docker/Podman runs as a different user.

Other changes:
* Detect Git commit
* Fixed log levels (stdout/stderr can be used for different purposes)
* Stop container on termination
* Allow to use a prebuilt repository cache
* Migrated to ubi9 where possible

* Provide DataDog helper scripts (to test later)
* Use stdout_open instead of duplicating stderr_open
* Support Docker output format
* Improve metadata detection handling
* Run build as root (Docker seems to ignore USER when copying files)
* Updated integration test