Remove broken link to docker source code (elastic#10833)

* Remove link to old repo for docker source

* Remove dockergithub attribute

libbeat/docs/shared-beats-attributes.asciidoc

@@ -14,7 +14,7 @@
 :monitoringdoc: https://www.elastic.co/guide/en/elastic-stack-overview/{doc-branch}
 :dashboards: https://artifacts.elastic.co/downloads/beats/beats-dashboards/beats-dashboards-{stack-version}.zip
 :dockerimage: docker.elastic.co/beats/{beatname_lc}:{version}
-:dockergithub: https://github.com/elastic/beats-docker/tree/{doc-branch}
+:dockerconfig: https://raw.githubusercontent.com/elastic/beats/{doc-branch}/deploy/docker/{beatname_lc}.docker.yml
 :downloads: https://artifacts.elastic.co/downloads/beats
 :ES-version: {stack-version}
 :LS-version: {stack-version}

libbeat/docs/shared-docker.asciidoc

@@ -5,8 +5,7 @@ Docker images for {beatname_uc} are available from the Elastic Docker
 registry. The base image is https://hub.docker.com/_/centos/[centos:7].
 
 A list of all published Docker images and tags is available at
-https://www.docker.elastic.co[www.docker.elastic.co]. The source code is in
-{dockergithub}[GitHub].
+https://www.docker.elastic.co[www.docker.elastic.co]. 
 
 These images are free to use under the Elastic license. They contain open source 
 and free commercial features and access to paid commercial features.