DockerCPUEnvironment bechmark returns empty output

[jiivan]

Description

Golem Version: 6f09e11 (develop)

Golem-Messages version (leave empty if unsure): 3.9.0

Electron version (if used): not aplicable

OS [e.g. Windows 10 Pro]: Linux

Branch (if launched from source): task_payment_rpc

Mainnet/Testnet: n/a

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Description of the issue: Runtime is torn down on linux buildbot

A clear and concise description of what went wrong, in which component, when and where.

Actual result:

__init__.py 237 INFO Runtime torn down.

Screenshots:

________________________ TestIntegration.test_benchmark ________________________
self = <tests.golem.envs.docker.cpu.test_integration.TestIntegration testMethod=test_benchmark>
    @inlineCallbacks
    def test_benchmark(self):
        config = DockerCPUConfig(work_dir=Path(tempfile.gettempdir()))
        env = DockerCPUEnvironment(config)
        yield env.prepare()
    
        Whitelist.add(env.BENCHMARK_IMAGE.split('/')[0])
>       score = yield env.run_benchmark()
tests/golem/envs/docker/cpu/test_integration.py:91: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
.venv/lib/python3.6/site-packages/twisted/internet/defer.py:1386: in _inlineCallbacks
    result = g.send(result)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
self = <golem.envs.docker.cpu.DockerCPUEnvironment object at 0x7f3dd83ac3c8>
    @inlineCallbacks
    def run_benchmark(self) -> Deferred:
        image, tag = self.BENCHMARK_IMAGE.split(':')
        yield self.install_prerequisites(DockerPrerequisites(
            image=image,
            tag=tag,
        ))
        payload = DockerPayload(
            image=image,
            tag=tag,
            user=None if is_windows() else str(os.getuid()),
            env={},
        )
        runtime = self.runtime(payload)
        yield runtime.prepare()
        yield runtime.start()
        yield runtime.wait_until_stopped()
        # Benchmark is supposed to output a single line containing a float value
        stdout = list(runtime.stdout('utf-8'))
        yield runtime.clean_up()
>       return float(stdout[0])
E       IndexError: list index out of range
golem/envs/docker/cpu.py:537: IndexError

Steps To Reproduce

Go to: https://buildbot.golem.network/buildbot/#/builders/9/builds/12938/steps/7/logs/stdio

Expected behavior

No errors

Logs and any additional context

Possibly related to #4185 or #4372

Proposed Solution?