copy a file from local filesystem into a container:
cat [local file path] | kubectl exec -i -n [namespace] [pod] -c [container] "--" sh -c "cat > [remote file path]"
ex:
cat keycloak-benchmark-dataset-0.10-SNAPSHOT.jar | k exec -i pods/keycloak-65f866dc7b-2kpn5 "--" sh -c "cat > /opt/keycloak/providers/keycloak-benchmark-dataset-0.10-SNAPSHOT.jar"