fix CI

Dockerfile

-FROM ubuntu:20.04
+FROM pandoc/core
 
-RUN --mount=type=cache,sharing=locked,target=/var/cache/apt --mount=type=cache,sharing=locked,target=/var/lib/apt \
-    apt-get update -qqy && DEBIAN_FRONTEND=noninteractive apt-get install -qqy \
-    pandoc \
-    pandoc-citeproc \
-    python-is-python3 \
-    python3-pip \
-    make \
- && python -m pip install --user --no-cache-dir \
-    'panflute<2'
+ENTRYPOINT []
+
+RUN apk --no-cache add make py3-pip \
+ && pip install panflute

cite.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 """
 Pandoc filter to include a raw bib citation
 """