Pre-built base images for external tests (reduce per-run build time and failures)

Every time an external test runs today, build_image.sh invokes dockerfile_functions.sh to dynamically generate a full Dockerfile and build it from scratch — including downloading Maven, compiling CRIU from source, installing SBT with GPG verification, etc. These tool-installation layers are identical across all runs of the same test and never change between invocations.

This proposal introduces a small set of pre-built base images published to ghcr.io/adoptium (GitHub Container Registry, https://github.com/orgs/adoptium/packages) so each test only needs a thin ~15-line Dockerfile that does FROM and adds the test-specific git clone + COPY test.sh.

Start by reading build_image.sh and dockerfile_functions.sh to trace how external-test Dockerfiles are generated and which tool-installation layers are rebuilt. Compare the current generated Dockerfile with the proposed thin form. Done means the shared base images are published to ghcr.io/adoptium and external tests use them while retaining test-specific cloning and COPY steps.

Written by the indexing model from the issue text.