ANN: new Docker image tagging scheme

[Albert Krewinkel <albert+pandoc-9EawChwDxG8hFhg+JK9F0w@public.gmane.org>]

I'm pleased to announce the release of new pandoc Docker images for
pandoc 2.17, as well as a new image tagging scheme:

-   The list of supported image tags is now explicitly listed in the
    Docker Hub repository readme.  For logistical reasons, we will
    support not more that around six or seven released versions.

-   Version tags are now "rolling", meaning that numerical versions will
    always point to the latest release that has the given version
    prefix. E.g., the tag `2.16` points to `2.16.2.0`. This is a common
    convention for Docker images. Use a four-part version tag to point
    to a specific release, e.g. `2.17.0.0`.

-   The `pandoc-crossref` filter is now available in `pandoc/core`
    images, as is a image library used to convert SVG images. Use the
    new `pandoc/minimal` repository for truly minimal imges.

-   A new stack "static" was introduced for `pandoc/minimal` images. The
    images contain just a statically compiled pandoc binary, but no OS.
    This is especially helpful when including pandoc into other Docker
    containers, e.g.

        COPY --from=pandoc/minimal:latest /pandoc /usr/bin/pandoc

-   The OS of an image can be controlled by appending the identifier to
    the version tag. E.g., to get the newest release in a pandoc+LaTeX
    image, use `pandoc/latex:latest-ubuntu`.

-   The repositories `pandoc/alpine`, `pandoc/ubuntu`,
    `pandoc/alpine-crossref`, `pandoc/ubuntu-crossref`,
    `pandoc/alpine-latex`, and `pandoc/ubuntu-latex` are deprecated and
    will no longer receive updates or new versions. Please use one of
    the three supported repositories with a respective tag.

Please refer to the image repositories for additional info:

- minimal: <https://hub.docker.com/r/pandoc/minimal>
- core: <https://hub.docker.com/r/pandoc/core>
- latex: <https://hub.docker.com/r/pandoc/latex>


Many thanks to Stephen McDowell and Caleb Maclennan for their continued
help and support; maintaining Docker images would not be possible
without their help!

-- 
Albert Krewinkel
GPG: 8eed e3e2 e8c5 6f18 81fe  e836 388d c0b2 1f63 1124