Update Dockerfile: Add cairo & pngquant

Author: Kjuly

Dockerfile

@@ -5,7 +5,10 @@ LABEL org.opencontainers.image.authors="dev@kjuly.com" \
       org.opencontainers.image.source=https://github.com/Kjuly/docker-mkdocs \
       org.opencontainers.image.description="A tiny Docker image for MkDocs, based on Alpine Linux with Python3."
 
-RUN apk add --no-cache py3-pip && \
+# The libraries for image processing (cairo, pngquant) are entirely optional,
+#   and only need to be installed if you want to use the social plugin or the optimize plugin.
+# Refter to https://squidfunk.github.io/mkdocs-material/plugins/requirements/image-processing
+RUN apk add --no-cache py3-pip cairo pngquant && \
     python3 -m venv .venv && source .venv/bin/activate \
     pip install pip setuptools certifi urllib3 requests idna --upgrade && \
     pip install mkdocs && \