Add Fleet Server install steps on K8s and Docker (#1184)

* Add Fleet Server install steps on K8s and Docker

* fixup

* Update docs/en/ingest-management/elastic-agent/running-on-kubernetes-managed-by-fleet.asciidoc

* Update docs/en/ingest-management/elastic-agent/elastic-agent-container.asciidoc

Author: kilfoyle

docs/en/ingest-management/elastic-agent/elastic-agent-container.asciidoc

@@ -95,6 +95,18 @@ docker run --rm docker.elastic.co/elastic-agent/elastic-agent:{version} elastic-
 
 include::{ingest-docs-root}/docs/en/ingest-management/tab-widgets/run-agent-image/widget.asciidoc[]
 
+If you need to run {fleet-server} as well, adjust the `docker run` command above by adding these environment variables:
+
+[source,yaml]
+----
+  --env FLEET_SERVER_ENABLE=true \ <1>
+  --env FLEET_SERVER_ELASTICSEARCH_HOST=<elasticsearch-host> \ <2>
+  --env FLEET_SERVER_SERVICE_TOKEN=<service-token> <3>
+----
+<1> Set to `true` to bootstrap {fleet-server} on this {agent}. This automatically forces {fleet} enrollment as well.
+<2> The Elasticsearch host for Fleet Server to communicate with, for example `http://elasticsearch:9200`.
+<3> Service token to use for communication with {es} and {kib}.
+
 [TIP] 
 .Running {agent} on a read-only file system
 ==== 

docs/en/ingest-management/elastic-agent/running-on-kubernetes-managed-by-fleet.asciidoc

@@ -67,8 +67,22 @@ To specify different destination/credentials, change the following parameters in
 <5> The basic authentication username used to connect to {kib} and retrieve a `service_token` to enable {fleet}.
 <6> The basic authentication password used to connect to {kib} and retrieve a `service_token` to enable {fleet}.
 
-Refer to <<agent-environment-variables>> for all available options.
+If you need to run {fleet-server} as well, adjust the `docker run` command above by adding these environment variables:
+
+[source,yaml]
+------------------------------------------------
+- name: FLEET_SERVER_ENABLE
+  value: "true" <1>
+- name: FLEET_SERVER_ELASTICSEARCH_HOST
+  value: "<elasticsearch-host>" <2>
+- name: FLEET_SERVER_SERVICE_TOKEN
+  value: "<service-token>" <3>
+------------------------------------------------
+<1> Set to `true` to bootstrap {fleet-server} on this {agent}. This automatically forces {fleet} enrollment as well.
+<2> The Elasticsearch host for Fleet Server to communicate with, for example `http://elasticsearch:9200`.
+<3> Service token to use for communication with {es} and {kib}.
 
+Refer to <<agent-environment-variables>> for all available options.
 
 [discrete]
 == Step 4: Configure tolerations