Fix DIND + Dependency Proxy docs

doc/user/packages/container_registry/index.md

@@ -345,7 +345,7 @@ Below is an example of what your `.gitlab-ci.yml` should look like:
 
 ```yaml
 build:
-  image: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/group/project/docker:19.03.12
+  image: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/docker:19.03.12
   services:
     - name: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/docker:18.09.7-dind
       alias: docker

doc/user/packages/dependency_proxy/index.md

@@ -266,7 +266,8 @@ error during connect: Get http://docker:2376/v1.39/info: dial tcp: lookup docker
 
 This can be resolved by setting a service alias for the Docker service:
 
-```plaintext
+```yaml
 services:
     - name: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/docker:18.09.7-dind
-      alias: docker```
+      alias: docker
+```