Fix Docker volume for new Kokoro instance #291

Job	Run time
build_attest_all (buildconfigs/key_xor_test_app.sh)	5m 6s
build_attest_all (buildconfigs/oak_client_android_app.sh)	15m 3s
build_attest_all (buildconfigs/oak_containers_agent.sh)	6m 25s
build_attest_all (buildconfigs/oak_containers_kernel.sh)	3m 26s
build_attest_all (buildconfigs/oak_containers_orchestrator.sh)	7m 37s
build_attest_all (buildconfigs/oak_containers_stage1.sh)	6m 38s
build_attest_all (buildconfigs/oak_containers_syslogd.sh)	8m 7s
build_attest_all (buildconfigs/oak_containers_system_image.sh)	14m 5s
build_attest_all (buildconfigs/oak_containers_nvidia_system_image.sh)	16m 49s
build_attest_all (buildconfigs/oak_echo_enclave_app.sh)	5m 2s
build_attest_all (buildconfigs/oak_echo_raw_enclave_app.sh)	5m 7s
build_attest_all (buildconfigs/oak_functions_enclave_app.sh)	5m 26s
build_attest_all (buildconfigs/oak_functions_insecure_enclave_app.sh)	5m 12s
build_attest_all (buildconfigs/oak_orchestrator.sh)	5m 8s
build_attest_all (buildconfigs/oak_restricted_kernel_simple_io_init_rd_wrapper_bin.sh)	5m 21s
build_attest_all (buildconfigs/stage0_bin.sh)	5m 27s
	1h 59m 59s

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