[CI] Pass SINGLE_TEST variable to docker container for isolating x-pack test runs to one test

Author: estolfo

.ci/docker/run-security

@@ -49,6 +49,7 @@ docker run \
   --env "ELASTIC_PASSWORD=changeme" \
   --env "ELASTIC_USER=elastic" \
   --env "TEST_SUITE=security" \
+  --env "SINGLE_TEST=${SINGLE_TEST}" \
   --volume $repo:/usr/src/app \
   --name elasticsearch-ruby \
   --rm \

.ci/run-tests

@@ -13,6 +13,7 @@ set -eo pipefail
 
 export RUBY_TEST_VERSION=${RUBY_TEST_VERSION:-2.6.1}
 export ELASTICSEARCH_VERSION=${ELASTICSEARCH_VERSION:-6.7.0}
+export SINGLE_TEST=${SINGLE_TEST}
 
 set +x
 export VAULT_TOKEN=$(vault write -field=token auth/approle/login role_id="$VAULT_ROLE_ID" secret_id="$VAULT_SECRET_ID")