[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/conftest.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+set -euo pipefail
+set -x
+
+if [[ -n $BUILDKITE_PULL_REQUEST && $BUILDKITE_PULL_REQUEST != "false" ]]
+then
+  is_pull_request=1
+  export BRANCH_NAME=PR-$BUILDKITE_PULL_REQUEST
+else
+  is_pull_request=0
+  export BRANCH_NAME=$BUILDKITE_BRANCH
+fi
+
+if [[ $BUILDKITE_BRANCH == "master" || $BUILDKITE_BRANCH == "release_"* ]]
+then
+  is_release_branch=1
+else
+  is_release_branch=0
+fi
+
+set +x