Add docs about CI_SERVER_HOST env variable

Signed-off-by: Balasankar "Balu" C <balasankar@gitlab.com>

doc/ci/variables/README.md

@@ -273,6 +273,7 @@ export CI_RUNNER_ID="10"
 export CI_RUNNER_DESCRIPTION="my runner"
 export CI_RUNNER_TAGS="docker, linux"
 export CI_SERVER="yes"
+export CI_SERVER_HOST="example.com"
 export CI_SERVER_NAME="GitLab"
 export CI_SERVER_REVISION="70606bf"
 export CI_SERVER_VERSION="8.9.0"
@@ -644,6 +645,8 @@ Running on runner-8a2f473d-project-1796893-concurrent-0 via runner-8a2f473d-mach
 ++ CI_PROJECT_DIR=/builds/gitlab-examples/ci-debug-trace
 ++ export CI_SERVER=yes
 ++ CI_SERVER=yes
+++ export 'CI_SERVER_HOST=example.com'
+++ CI_SERVER_HOST='example.com'
 ++ export 'CI_SERVER_NAME=GitLab CI'
 ++ CI_SERVER_NAME='GitLab CI'
 ++ export CI_SERVER_VERSION=
@@ -678,6 +681,8 @@ Running on runner-8a2f473d-project-1796893-concurrent-0 via runner-8a2f473d-mach
 ++ CI_JOB_NAME=debug_trace
 ++ export CI_JOB_STAGE=test
 ++ CI_JOB_STAGE=test
+++ export CI_SERVER_HOST=example.com
+++ CI_SERVER_HOST=example.com
 ++ export CI_SERVER_NAME=GitLab
 ++ CI_SERVER_NAME=GitLab
 ++ export CI_SERVER_VERSION=8.14.3-ee

doc/ci/variables/predefined_variables.md

@@ -101,6 +101,7 @@ future GitLab releases.**
 | `CI_RUNNER_TAGS`                        | 8.10   | 0.5    | The defined runner tags |
 | `CI_RUNNER_VERSION`                     | all    | 10.6   | GitLab Runner version that is executing the current job |
 | `CI_SERVER`                             | all    | all    | Mark that job is executed in CI environment |
+| `CI_SERVER_HOST`                        | 12.1   | all    | Host component of the GitLab instance URL, without protocol and port (like gitlab.example.com) |
 | `CI_SERVER_NAME`                        | all    | all    | The name of CI server that is used to coordinate jobs |
 | `CI_SERVER_REVISION`                    | all    | all    | GitLab revision that is used to schedule jobs |
 | `CI_SERVER_VERSION`                     | all    | all    | GitLab version that is used to schedule jobs |