More CI improvements (#8313)

* Reduce clutter in log of Python test

* Set up BuildKite test analytics

* Add separate step for building containers

* Enable incremental update of CI stack; custom agent IAM policy

tests/buildkite/build-containers.sh

@@ -0,0 +1,36 @@
+#!/bin/bash
+
+set -euo pipefail
+set -x
+
+if [ "$#" -lt 1 ]
+then
+  echo "Usage: $0 [container to build]"
+  return 1
+fi
+container=$1
+
+source tests/buildkite/conftest.sh
+
+echo "--- Build container ${container}"
+
+BUILD_ARGS=""
+
+case "${container}" in
+  gpu|rmm)
+    BUILD_ARGS="$BUILD_ARGS --build-arg CUDA_VERSION_ARG=$CUDA_VERSION"
+    BUILD_ARGS="$BUILD_ARGS --build-arg RAPIDS_VERSION_ARG=$RAPIDS_VERSION"
+    ;;
+
+  jvm_gpu_build)
+    BUILD_ARGS="$BUILD_ARGS --build-arg CUDA_VERSION_ARG=$CUDA_VERSION"
+    ;;
+
+  *)
+    echo "Unrecognized container ID: ${container}"
+    return 2
+    ;;
+esac
+
+# Run a no-op command. This will simply build the container and push it to the private registry
+tests/ci_build/ci_build.sh ${container} docker ${BUILD_ARGS} bash