[CI] Sign in to Docker Hub for Buildkite pipelines

Unauthenticated pulls from Docker Hub are heavily rate-limited. After
this change, we will use authenticated Docker Hub user so that we can
pull public images for tests without being rate-limited.

Author: jozala

.buildkite/hooks/pre-command

@@ -94,6 +94,13 @@ if [[ "${USE_PROD_DOCKER_CREDENTIALS:-}" == "true" ]]; then
   fi
 fi
 
+if which docker > /dev/null 2>&1; then
+  DOCKERHUB_REGISTRY_USERNAME="$(vault read -field=username secret/ci/elastic-elasticsearch/docker_hub_public_ro_credentials)"
+  DOCKERHUB_REGISTRY_PASSWORD="$(vault read -field=password secret/ci/elastic-elasticsearch/docker_hub_public_ro_credentials)"
+
+  docker login --username "$DOCKERHUB_REGISTRY_USERNAME" --password "$DOCKERHUB_REGISTRY_PASSWORD" docker.io
+fi
+
 if [[ "$BUILDKITE_AGENT_META_DATA_PROVIDER" != *"k8s"* ]]; then
   # Run in the background, while the job continues
   nohup .buildkite/scripts/setup-monitoring.sh </dev/null >/dev/null 2>&1 &