Add GitLab Runner Helm Chart documenation for cucstom certificates

This outlines how to provide the custom ssl certificate to the runner
for accessing GitLab in the case that GitLab is using a
custom/self-signed certificate.

doc/install/kubernetes/gitlab_runner_chart.md

@@ -54,6 +54,13 @@ gitlabURL: http://gitlab.your-domain.com/
 ##
 runnerRegistrationToken: ""
 
+## Set the certsSecretName in order to pass custom certficates for GitLab Runner to use
+## Provide resource name for a Kubernetes Secret Object in the same namespace,
+## this is used to populate the /etc/gitlab-runner/certs directory
+## ref: https://docs.gitlab.com/runner/configuration/tls-self-signed.html#supported-options-for-self-signed-certificates
+##
+#certsSecretName:
+
 ## Configure the maximum number of concurrent jobs
 ## ref: https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-global-section
 ##
@@ -135,6 +142,52 @@ runners:
   privileged: true
 ```
 
+### Providing a custom certificate for accessing GitLab
+
+You can provide a [Kubernetes Secret](https://kubernetes.io/docs/concepts/configuration/secret/)
+to the GitLab Runner Helm Chart, which will be used to populate the container's
+`/etc/gitlab-runner/certs` directory.
+
+Each key name in the Secret will be used as a filename in the directory, with the
+file content being the value associated with the key.
+
+More information on how GitLab Runner uses these certificates can be found in the
+[Runner Documentation](https://docs.gitlab.com/runner/configuration/tls-self-signed.html#supported-options-for-self-signed-certificates).
+
+ - The key/file name used should be in the format `<gitlab-hostname>.crt`. For example: `gitlab.your-domain.com.crt`.
+ - Any intermediate certificates need to be concatenated to your server certificate in the same file.
+ - The hostname used should be the one the certificate is registered for.
+
+The GitLab Runner Helm Chart does not create a secret for you. In order to create
+the secret, you can prepare your certificate on you local machine, and then run
+the `kubectl create secret` command from the directory with the certificate
+
+```bash
+kubectl
+  --namespace <NAMESPACE>
+  create secret generic <SECRET_NAME>
+  --from-file=<CERTFICATE_FILENAME>
+```
+
+- `<NAMESPACE>` is the Kubernetes namespace where you want to install the GitLab Runner.
+- `<SECRET_NAME>` is the Kubernetes Secret resource name. For example: `gitlab-domain-cert`
+- `<CERTFICATE_FILENAME>` is the filename for the certificate in your current directory that will be imported into the secret
+
+You then need to provide the secret's name to the GitLab Runner chart.
+
+Add the following to your `values.yaml`
+
+```yaml
+## Set the certsSecretName in order to pass custom certficates for GitLab Runner to use
+## Provide resource name for a Kubernetes Secret Object in the same namespace,
+## this is used to populate the /etc/gitlab-runner/certs directory
+## ref: https://docs.gitlab.com/runner/configuration/tls-self-signed.html#supported-options-for-self-signed-certificates
+##
+certsSecretName: <SECRET NAME>
+```
+
+- `<SECRET_NAME>` is the Kubernetes Secret resource name. For example: `gitlab-domain-cert`
+
 ## Installing GitLab Runner using the Helm Chart
 
 Once you [have configured](#configuration) GitLab Runner in your `values.yml` file,