Yamllint fixes (#4410)

* Lint everything in the repository with yamllint

* yamllint fixes: syntax fixes only

* yamllint fixes: move comments to play names

* yamllint fixes: indent comments in .gitlab-ci.yml file

inventory/sample/group_vars/all/docker.yml

@@ -1,13 +1,14 @@
+---
 ## Uncomment this if you want to force overlay/overlay2 as docker storage driver
 ## Please note that overlay2 is only supported on newer kernels
-#docker_storage_options: -s overlay2
+# docker_storage_options: -s overlay2
 
 ## Enable docker_container_storage_setup, it will configure devicemapper driver on Centos7 or RedHat7.
 docker_container_storage_setup: false
 
 ## It must be define a disk path for docker_container_storage_setup_devs.
 ## Otherwise docker-storage-setup will be executed incorrectly.
-#docker_container_storage_setup_devs: /dev/vdb
+# docker_container_storage_setup_devs: /dev/vdb
 
 ## Uncomment this if you have more than 3 nameservers, then we'll only use the first 3.
 docker_dns_servers_strict: false
@@ -32,12 +33,12 @@ docker_rpm_keepcache: 0
 ## An obvious use case is allowing insecure-registry access to self hosted registries.
 ## Can be ipaddress and domain_name.
 ## example define 172.19.16.11 or mirror.registry.io
-#docker_insecure_registries:
+# docker_insecure_registries:
 #   - mirror.registry.io
 #   - 172.19.16.11
 
 ## Add other registry,example China registry mirror.
-#docker_registry_mirrors:
+# docker_registry_mirrors:
 #   - https://registry.docker-cn.com
 #   - https://mirror.aliyuncs.com
 
@@ -46,7 +47,7 @@ docker_rpm_keepcache: 0
 ## or private, which control whether mounts in the file system
 ## namespace set up for docker will receive or propagate mounts
 ## and unmounts. Leave empty for system default
-#docker_mount_flags:
+# docker_mount_flags:
 
 ## A string of extra options to pass to the docker daemon.
 ## This string should be exactly as you wish it to appear.