Dockerfile: add missing zstd dependency

Signed-off-by: Arthur Zamarin <arthurzam@gentoo.org>

Author: arthurzam

Dockerfile

@@ -1,7 +1,7 @@
 FROM python:3-slim
 ARG PKGCHECK_VERSION
 
-RUN apt-get update && apt-get install -y git && \
+RUN apt-get update && apt-get install -y git zstd && \
     rm -rf /var/lib/apt/lists/ /var/cache/apt /var/cache/dpkg && \
     pip install pkgcheck==${PKGCHECK_VERSION} setuptools requests && \
     pip cache purge