upstream CI: Build containers in parallel jobs

In the current build container pipeline, all steps are serialized in a
single job, and if one of the jobs fail to build, due to broken
dependent image, or some Azure glitch, like slow connection, the only
way to rebuild the failed container is to rebuild all containers.

By building containers in parallel jobs, if a container fails to build
it is possible to restart only the failed job.

tests/azure/build-containers.yml

@@ -13,34 +13,49 @@ trigger: none
 pool:
   vmImage: 'ubuntu-20.04'
 
-jobs:
+stages:
 
-- template: templates/build_container.yml
-  parameters:
-    job_name_suffix: Centos7
-    container_name: centos-7
-    build_scenario_name: centos-7-build
+- stage: CentOS_7
+  dependsOn: []
+  jobs:
+  - template: templates/build_container.yml
+    parameters:
+      job_name_suffix: Centos7
+      container_name: centos-7
+      build_scenario_name: centos-7-build
 
-- template: templates/build_container.yml
-  parameters:
-    job_name_suffix: C8S
-    container_name: c8s
-    build_scenario_name: c8s-build
+- stage: CentOS_8_Stream
+  dependsOn: []
+  jobs:
+  - template: templates/build_container.yml
+    parameters:
+      job_name_suffix: C8S
+      container_name: c8s
+      build_scenario_name: c8s-build
 
-- template: templates/build_container.yml
-  parameters:
-    job_name_suffix: C9S
-    container_name: c9s
-    build_scenario_name: c9s-build
+- stage: CentOS_9_Stream
+  dependsOn: []
+  jobs:
+  - template: templates/build_container.yml
+    parameters:
+      job_name_suffix: C9S
+      container_name: c9s
+      build_scenario_name: c9s-build
 
-- template: templates/build_container.yml
-  parameters:
-    job_name_suffix: FedoraLatest
-    container_name: fedora-latest
-    build_scenario_name: fedora-latest-build
+- stage: Fedora_Latest
+  dependsOn: []
+  jobs:
+  - template: templates/build_container.yml
+    parameters:
+      job_name_suffix: FedoraLatest
+      container_name: fedora-latest
+      build_scenario_name: fedora-latest-build
 
-- template: templates/build_container.yml
-  parameters:
-    job_name_suffix: FedoraRawhide
-    container_name: fedora-rawhide
-    build_scenario_name: fedora-rawhide-build
+- stage: Fedora_Rawhide
+  dependsOn: []
+  jobs:
+  - template: templates/build_container.yml
+    parameters:
+      job_name_suffix: FedoraRawhide
+      container_name: fedora-rawhide
+      build_scenario_name: fedora-rawhide-build