Docker: change plugin path in custom docker (#21837)

This is to make the custom dockerfiles compatible with our instruction
to use persitent volumes for the /var/lib/grafana dir. Without this
change the contents of that folder will be overshadowed by the persitent
volume and none of the pre-installed plugins will be available.

packaging/docker/custom/ubuntu.Dockerfile

@@ -1,4 +1,4 @@
-ARG GRAFANA_VERSION="latest-ubuntu"
+ARG GRAFANA_VERSION="latest"
 
 FROM grafana/grafana:${GRAFANA_VERSION}-ubuntu
 
@@ -9,6 +9,11 @@ ARG DEBIAN_FRONTEND=noninteractive
 
 ARG GF_INSTALL_IMAGE_RENDERER_PLUGIN="false"
 
+ENV GF_PATHS_PLUGINS="/var/lib/grafana-plugins"
+
+RUN mkdir -p "$GF_PATHS_PLUGINS" && \
+    chown -R grafana:grafana "$GF_PATHS_PLUGINS"
+
 RUN if [ $GF_INSTALL_IMAGE_RENDERER_PLUGIN = "true" ]; then \
     apt-get update && \
     apt-get upgrade -y && \