Provide merging details and example for YAML Anchors

doc/ci/yaml/README.md

@@ -4156,7 +4156,8 @@ Read more about the various [YAML features](https://learnxinyminutes.com/docs/ya
 YAML has a handy feature called 'anchors', which lets you easily duplicate
 content across your document. Anchors can be used to duplicate/inherit
 properties, and is a perfect example to be used with [hidden jobs](#hide-jobs)
-to provide templates for your jobs.
+to provide templates for your jobs. When there is duplicate keys, GitLab will
+perform a reverse deep merge based on the keys.
 
 The following example uses anchors and map merging. It will create two jobs,
 `test1` and `test2`, that will inherit the parameters of `.job_template`, each
@@ -4217,6 +4218,8 @@ directive defined in `.postgres_services` and `.mysql_services` respectively:
 .job_template: &job_definition
   script:
     - test project
+  tags:
+    - dev
 
 .postgres_services:
   services: &postgres_definition
@@ -4231,6 +4234,8 @@ directive defined in `.postgres_services` and `.mysql_services` respectively:
 test:postgres:
   <<: *job_definition
   services: *postgres_definition
+  tags:
+    - postgres
 
 test:mysql:
   <<: *job_definition
@@ -4243,6 +4248,8 @@ The expanded version looks like this:
 .job_template:
   script:
     - test project
+  tags:
+    - dev
 
 .postgres_services:
   services:
@@ -4260,6 +4267,8 @@ test:postgres:
   services:
     - postgres
     - ruby
+  tags:
+    - postgres
 
 test:mysql:
   script:
@@ -4267,10 +4276,15 @@ test:mysql:
   services:
     - mysql
     - ruby
+  tags:
+    - dev
 ```
 
 You can see that the hidden jobs are conveniently used as templates.
 
+NOTE: **Note:**
+Note that `tags: [dev]` has been overwritten by `tags: [postgres]`.
+
 NOTE: **Note:**
 You can't use YAML anchors across multiple files when leveraging the [`include`](#include)
 feature. Anchors are only valid within the file they were defined in.