[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-gpu-rpkg.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+set -euo pipefail
+
+CUDA_VERSION=11.0.3
+
+source tests/buildkite/conftest.sh
+
+echo "--- Build XGBoost R package with CUDA"
+
+if [[ ($is_pull_request == 0) && ($is_release_branch == 1) ]]
+then
+
+tests/ci_build/ci_build.sh gpu_build_r_centos7 docker \
+  --build-arg CUDA_VERSION_ARG=${CUDA_VERSION} tests/ci_build/build_r_pkg_with_cuda.sh \
+  ${BUILDKITE_COMMIT}
+
+echo "--- Upload R tarball"
+aws s3 cp xgboost_r_gpu_linux_*.tar.gz s3://xgboost-nightly-builds/${BRANCH_NAME}/ \
+  --acl public-read --no-progress
+
+fi