WeasyPrint install instructions for Alpine Linux

Finally! With the latest versions of alpine & weasyprint we can keep build pretty small & clean.

Here my current approach:

FROM python:3.7-alpine
ENV PYTHONUNBUFFERED 1

RUN apk --update --upgrade --no-cache add \
    cairo-dev pango-dev gdk-pixbuf

...

ADD /etc/pip/requirements.txt /code/requirements.txt
RUN set -ex \
    && apk add --no-cache --virtual .build-deps \
        musl-dev gcc jpeg-dev zlib-dev libffi-dev \
    && pip install --no-cache-dir -r /code/requirements.txt \
    && apk del .build-deps

...

Packages cairo-dev pango-dev gdk-pixbuf need in runtime, so we kept them. Other ones need only on building stage - so we clean.

In requirements.txt just weasyprint and my specific dependencies, not related to this issue.

And this is also documented which I somehow overread 🙄