Add troubleshooting steps for Secret Detection failure

doc/user/application_security/secret_detection/index.md

@@ -283,3 +283,25 @@ Support for custom certificate authorities was introduced in the following versi
 ### Getting warning message `gl-secret-detection-report.json: no matching files`
 
 For information on this, see the [general Application Security troubleshooting section](../../../ci/pipelines/job_artifacts.md#error-message-no-files-to-upload).
+
+### Error: `Couldn't run the gitleaks command: exit status 2`
+
+This error is usually caused by the `GIT_DEPTH` value of 50 that is set for all [projects by default](../../../ci/pipelines/settings.md#git-shallow-clone). 
+
+For example, if a pipeline is triggered from a Merge Request containing 60 commits while the `GIT_DEPTH` is set to 50, the Secret Detection job will fail as the clone will not have been deep enough to contain all of the relevant commits. 
+
+You can confirm this to be the cause of the error by implementing a [logging level](../../application_security/secret_detection/index.md#logging-level) of `debug`. Once implemented, the logs should look similar to the following example, wherein an "object not found" error can be seen:
+
+```plaintext
+ERRO[2020-11-18T18:05:52Z] object not found                             
+[ERRO] [secrets] [2020-11-18T18:05:52Z] ▶ Couldn't run the gitleaks command: exit status 2
+[ERRO] [secrets] [2020-11-18T18:05:52Z] ▶ Gitleaks analysis failed: exit status 2
+```
+
+If this is the case, we can resolve the issue by setting the [`GIT_DEPTH` variable](../../../ci/runners/README.md#shallow-cloning) to a higher value. In order to apply this only to the Secret Detection job, the following can be added to your `.gitlab-ci.yml`:
+
+```yaml
+secret_detection:
+  variables:
+    GIT_DEPTH: 100
+```