Merge branch 'docs-shellcheck' into 'master'

Shellcheck - Use stable image

See merge request gitlab-org/gitlab!20681

doc/development/shell_scripting_guide/index.md

@@ -60,7 +60,7 @@ All projects with shell scripts should use this GitLab CI/CD job:
 
 ```yaml
 shell check:
-  image: koalaman/shellcheck-alpine
+  image: koalaman/shellcheck-alpine:stable
   stage: test
   before_script:
     - shellcheck --version