[CI] Migrate CI pipelines from Jenkins to BuildKite (#8142)

* [CI] Migrate CI pipelines from Jenkins to BuildKite

* Require manual approval

* Less verbose output when pulling Docker

* Remove us-east-2 from metadata.py

* Add documentation

* Add missing underscore

* Add missing punctuation

* More specific instruction

* Better paragraph structure

tests/buildkite/build-jvm-packages-gpu.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+
+set -euo pipefail
+
+SPARK_VERSION=3.0.1
+CUDA_VERSION=11.0.3
+
+source tests/buildkite/conftest.sh
+
+echo "--- Build XGBoost JVM packages with CUDA"
+
+if [[ ($is_pull_request == 1) || ($is_release_branch == 0) ]]
+then
+  arch_flag="-DGPU_COMPUTE_VER=75"
+else
+  arch_flag=""
+fi
+
+tests/ci_build/ci_build.sh jvm_gpu_build nvidia-docker \
+  --build-arg CUDA_VERSION_ARG=${CUDA_VERSION} tests/ci_build/build_jvm_packages.sh \
+  ${SPARK_VERSION} -Duse.cuda=ON ${arch_flag}