Adding Cairo support

[jurgenhaas]

When e.g. using the mkdocs-material theme, there is image processing recommended, e.g. for their "social" plugin. Details see at https://squidfunk.github.io/mkdocs-material/plugins/requirements/image-processing and https://squidfunk.github.io/mkdocs-material/plugins/social

This requires 3 more packages to be installed by apk in the docker image:

• cairo
• freetype-dev
• jpeg-dev

Could they be added please?

[Cevedale]

i think i have an idea...