integration-cli: getContainerCount() fix trimming prefix
caught by staticcheck:
```
integration-cli/docker_utils_test.go:66:29: SA1024: cutset contains duplicate characters (staticcheck)
```
@@ -63,7 +63,7 @@ func getContainerCount(c *check.C) int {
for _, line := range lines {
if strings.Contains(line, containers) {
output := strings.TrimSpace(line)
- output = strings.TrimLeft(output, containers)
+ output = strings.TrimPrefix(output, containers)
output = strings.Trim(output, " ")
containerCount, err := strconv.Atoi(output)
assert.NilError(c, err)
