<p>I have a helm chart that I was going to deploy and I would like to use the deployment it creates as a sidecar for another deployment. Is this possible using the Rancher's GUI or is it something that I can directly configure in the YAML?</p>

<p><strong>TL;DR:</strong> No</p>
<hr />
<p>Not really possible. You have to specify multiple containers in the same pod/deployment manifest to create sidecars. Like this:</p>
<pre class="lang-yaml prettyprint-override"><code>apiVersion: v1
kind: Pod
metadata:
  name: webserver
spec:
  volumes:
    - name: shared-logs
      emptyDir: {}

  containers:
    - name: nginx
      image: nginx
      volumeMounts:
        - name: shared-logs
          mountPath: /var/log/nginx

    - name: sidecar-container
      image: busybox
      command: [&quot;sh&quot;,&quot;-c&quot;,&quot;while true; do cat /var/log/nginx/access.log /var/log/nginx/error.log; sleep 30; done&quot;]
      volumeMounts:
        - name: shared-logs
          mountPath: /var/log/nginx
</code></pre>
<hr />
<p>Alternatively, you can achieve this by using <a href="https://kubernetes.io/docs/reference/access-authn-authz/admission-controllers/" rel="nofollow noreferrer">Admission Controllers</a>, the same way Istio does, but this is way outside of the scope of this question.</p>
