integration-cli: getContainerCount() fix trimming prefix

[thaJeztah]

splitting this from #39668

caught by staticcheck:

integration-cli/docker_utils_test.go:66:29: SA1024: cutset contains duplicate characters (staticcheck)

[thaJeztah]

rebased to cleanup old CI results

@kolyshkin @AkihiroSuda PTAL

[thaJeztah]

Windows RS1 failure: https://ci.docker.com/public/job/moby/job/PR-39819/2/execution/node/190/log/

Haven't seen that one before; looks like possible slowness on those machines?

11:18:05  --- FAIL: TestStateTimeoutWait (0.35s)
11:18:05      state_test.go:137: Stop callback doesn't fire in 200 milliseconds
11:18:05  FAIL

Windows RS5 failure is a known flaky: https://ci.docker.com/public/job/moby/job/PR-39819/2/execution/node/177/log/

11:45:31  FAIL: docker_cli_run_test.go:1792: DockerSuite.TestRunInteractiveWithRestartPolicy
11:45:31  
11:45:31  assertion failed: 
11:45:31  Command:  d:\CI-2\CI-02c9b0674\binary\docker.exe run -i --name test-inter-restart --restart=always busybox sh
11:45:31  ExitCode: 0
11:45:31  Stdout:   
11:45:31  Stderr:   
11:45:31  
11:45:31  Failures:
11:45:31  ExitCode was 0 expected 11