Set default, workflow, and use rules in CI config

Signed-off-by: Rémy Coutable <remy@rymai.me>

.gitlab-ci.yml

-image: golang:1.12
+workflow:
+  rules:
+    # For merge requests, create a pipeline.
+    - if: '$CI_MERGE_REQUEST_IID'
+    # For `master` branch, create a pipeline (this includes on schedules, pushes, merges, etc.).
+    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
+    # For tags, create a pipeline.
+    - if: '$CI_COMMIT_TAG'
+    # For stable branches, create a pipeline.
+    - if: '$CI_COMMIT_BRANCH =~ /^[\d-]+-stable$/'
+
+default:
+  image: golang:1.12
+  tags:
+    - gitlab-org
 
 # Disable DIND for SAST because we need to execute a before_script in the gosec-sast job
 variables:
@@ -8,7 +22,7 @@ verify:
   script:
   - make verify
 
-.test_template: &test_definition
+.test:
   services:
     - name: registry.gitlab.com/gitlab-org/build/cng/gitaly:latest
       # Disable the hooks so we don't have to stub the GitLab API
@@ -22,20 +36,20 @@ verify:
   - make test
 
 test using go 1.12:
+  extends: .test
   image: golang:1.12
-  <<: *test_definition
 
 test using go 1.13:
+  extends: .test
   image: golang:1.13
-  <<: *test_definition
 
 test using go 1.14:
+  extends: .test
   image: golang:1.14
-  <<: *test_definition
 
 test:release:
-  only:
-    - tags
+  rules:
+    - if: '$CI_COMMIT_TAG'
   script:
     - git describe --exact-match
 
@@ -47,7 +61,7 @@ gosec-sast:
   before_script:
     - apk add make
     - make install
-    
+
 code_navigation:
   image: registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6-golang-1.14-git-2.24
   allow_failure: true
@@ -59,4 +73,4 @@ code_navigation:
     - mv dump.lsif.tmp lsif
   artifacts:
     paths:
-      - lsif/
\ No newline at end of file
+      - lsif/