Plugin links to static images do not include configured subpaths

[sbruckmann]

What happened:
We are serving multiple Grafana instances from the same URL. To get this to work we are serving each Grafana instance from a different subpath and we use the serve_from_sub_path option in the config. All instances run on k8s and we use an ingress to route the traffic to the correct instance. With this config we expected the link that is generated by the rendering plugin when you try to share a panel to also include the configured subpath which is not the case.

What you expected to happen:
The generated link to the static rendered image includes the same subpath as the Grafana instance itself.

How to reproduce it (as minimally and precisely as possible):

• Build a custom docker image with the Dockerfile provided by the official Grafana repository and enable the GF_INSTALL_IMAGE_RENDERER_PLUGIN=true option.
• Run this image with GF_SERVER_ROOT_URL set to include a subpath (e.g. GF_SERVER_ROOT_URL=http://1.2.3.4:3000/first/second) and GF_SERVER_SERVE_FROM_SUB_PATH=true
• Login to the instance, create a dashboard and a panel and try to share it. The link to the static image that is generated by the plugin does not contain the configured subpath.
• If the root path of the used URL (e.g. http://1.2.3.4:3000/ ) gets routed to your instance too it still works since you get redirected to the correct subpath by Grafana, but that is sadly not the case for us.

Environment:

• Grafana Image Renderer version: 3.10.1
• Grafana version: 10.2.3
• Installed plugin or remote renderer service: installed plugin