Fix overwriting the SOURCE_CODE variable in Code Quality

This change fixes overwrite of SOURCE_CODE environment variable.

Changelog: fixed

doc/ci/testing/code_quality.md

@@ -254,7 +254,7 @@ Code Quality can be customized by defining available CI/CD variables:
 | `ENGINE_MEMORY_LIMIT_BYTES`     | Set the memory limit for engines. Default: 1,024,000,000 bytes. |
 | `REPORT_STDOUT`                 | Set to print the report to `STDOUT` instead of generating the usual report file. |
 | `REPORT_FORMAT`                 | Set to control the format of the generated report file. Either `json` or `html`. |
-| `SOURCE_CODE`                   | Path to the source code to scan. |
+| `SOURCE_CODE`                   | Path to the source code to scan. Must be the absolute path to a directory where cloned sources are stored. |
 | `TIMEOUT_SECONDS`               | Custom timeout per engine container for the `codeclimate analyze` command. Default: 900 seconds (15 minutes) |
 
 ## Output

lib/gitlab/ci/templates/Jobs/Code-Quality.gitlab-ci.yml

@@ -15,7 +15,7 @@ code_quality:
     DOCKER_SOCKET_PATH: /var/run/docker.sock
   needs: []
   script:
-    - export SOURCE_CODE=$PWD
+    - export SOURCE_CODE=${SOURCE_CODE:-$PWD}
     - |
       if ! docker info &>/dev/null; then
         if [ -z "$DOCKER_HOST" ] && [ -n "$KUBERNETES_PORT" ]; then