Convention B (Docker-Hub-style <service>:<full-version>) completes the
gradle+oraclejdk migration. Workflow flips images/tags/subject-name from
gradle-8 to gradle; 8 major-line alias added. Dockerfile FROM now
references the new oraclejdk:8u201 package name from the prior commit;
LABEL maintainer was missing and is added alongside 5 OCI image-spec
keys. Both metadata-action labels: block and Dockerfile LABEL lines
carry the same values so manual and CI builds match.
Sequencing: must land after the prior commit's CI publishes
ghcr.io/tiennm99/oraclejdk:8u201, otherwise the gradle CI build will
fail until oraclejdk re-publishes.
tiennm99