Add highlighting to compose-file reference

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

_includes/content/compose-extfields-sub.md

@@ -8,7 +8,7 @@ your Compose file and their name start with the `x-` character sequence.
 > (for the 2.x series), extension fields are also allowed at the root
 > of service, volume, network, config and secret definitions.
 
-```none
+```yaml
 version: '3.4'
 x-custom:
   items:
@@ -24,7 +24,7 @@ inserted in your resource definitions using [YAML anchors](http://www.yaml.org/s
 For example, if you want several of your services to use the same logging
 configuration:
 
-```none
+```yaml
 logging:
   options:
     max-size: '12m'
@@ -34,7 +34,7 @@ logging:
 
 You may write your Compose file as follows:
 
-```none
+```yaml
 version: '3.4'
 x-logging:
   &default-logging
@@ -55,7 +55,7 @@ services:
 It is also possible to partially override values in extension fields using
 the [YAML merge type](http://yaml.org/type/merge.html). For example:
 
-```none
+```yaml
 version: '3.4'
 x-volumes:
   &default-volume

_includes/content/compose-var-sub.md

@@ -3,8 +3,10 @@ variable values from the shell environment in which `docker-compose` is run. For
 example, suppose the shell contains `POSTGRES_VERSION=9.3` and you supply this
 configuration:
 
-    db:
-      image: "postgres:${POSTGRES_VERSION}"
+```yaml
+db:
+  image: "postgres:${POSTGRES_VERSION}"
+```
 
 When you run `docker-compose up` with this configuration, Compose looks for the
 `POSTGRES_VERSION` environment variable in the shell and substitutes its value
@@ -47,9 +49,11 @@ dollar sign. This also prevents Compose from interpolating a value, so a `$$`
 allows you to refer to environment variables that you don't want processed by
 Compose.
 
-    web:
-      build: .
-      command: "$$VAR_NOT_INTERPOLATED_BY_COMPOSE"
+```yaml
+web:
+  build: .
+  command: "$$VAR_NOT_INTERPOLATED_BY_COMPOSE"
+```
 
 If you forget and use a single dollar sign (`$`), Compose interprets the value
 as an environment variable and warns you: