Currently Jenkinsfile Runner starts internal builds in random places defined by the TemporaryDirectoryAllocator. It had some sense when JFR was inheriting from JTH/JUnit, but now it is no longer needed.

I propose the following changes:

• Keep a random TMP_DIR allocator in JFR by default, so that it can run in non-Docker mode. But change the path to make the usages of JFR explicit
• Allow passing a custom workspace. Now it can be done via "-Djenkins.model.Jenkins.workspacesDir=/workspace/build", but it would be nice to have an option to automate it
• Update Docker images to use the fixed Workspace path by default, add volume