Docker: Custom dockerfiles, docker and image rendering docs update (#20492)

Adds support for using custom dockerfiles to pre-install image 
renderer plugins.
Updates docs for docker and image rendering.

Fixes #20241

Co-Authored-By: Leonard Gram <leo@xlson.com>

packaging/docker/custom/Dockerfile

@@ -2,8 +2,31 @@ ARG GRAFANA_VERSION="latest"
 
 FROM grafana/grafana:${GRAFANA_VERSION}
 
+USER root
+
+ARG GF_INSTALL_IMAGE_RENDERER_PLUGIN="false"
+
+RUN if [ $GF_INSTALL_IMAGE_RENDERER_PLUGIN = "true" ]; then \
+    echo "http://dl-cdn.alpinelinux.org/alpine/edge/community" >> /etc/apk/repositories && \
+    echo "http://dl-cdn.alpinelinux.org/alpine/edge/main" >> /etc/apk/repositories && \
+    echo "http://dl-cdn.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories && \
+    apk --no-cache  upgrade && \
+    apk add --no-cache udev ttf-opensans chromium && \
+    rm -rf /tmp/* && \
+    rm -rf /usr/share/grafana/tools/phantomjs; \
+fi
+
 USER grafana
 
+ENV GF_RENDERER_PLUGIN_CHROME_BIN="/usr/bin/chromium-browser"
+
+RUN if [ $GF_INSTALL_IMAGE_RENDERER_PLUGIN = "true" ]; then \
+    grafana-cli \
+        --pluginsDir "$GF_PATHS_PLUGINS" \
+        --pluginUrl https://github.com/grafana/grafana-image-renderer/releases/latest/download/plugin-linux-x64-glibc-no-chromium.zip \
+        plugins install grafana-image-renderer; \
+fi
+
 ARG GF_INSTALL_PLUGINS=""
 
 RUN if [ ! -z "${GF_INSTALL_PLUGINS}" ]; then \