Enable buildkit cache for package manager

packaging/Dockerfile

@@ -1,3 +1,11 @@
+# syntax = docker/dockerfile:experimental
+#
+# Requires Docker v18.06 or later and BuildKit mode to use cache mount
+# Docker v18.06 also requires the daemon to be running in experimental mode.
+#
+# $ export DOCKER_BUILDKIT=1
+#
+# See https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/experimental.md
 #
 # To build for arm, see https://www.docker.com/blog/getting-started-with-docker-for-arm-on-linux/
 #
@@ -14,10 +22,14 @@ ARG VERSION
 RUN git clone --depth=1 -b $VERSION https://github.com/ruediger/VobSub2SRT /source
 
 FROM debian:buster AS build-debian
-RUN apt update && apt install -y libtiff5-dev libtesseract-dev tesseract-ocr-eng build-essential cmake pkg-config
+RUN --mount=type=cache,id=apt,target=/var/cache/apt \
+	apt update && apt install -y \
+	build-essential cmake libtesseract-dev libtiff5-dev pkg-config tesseract-ocr-eng
 
 FROM registry.gitlab.com/pld-linux/pld AS build-pld
-RUN poldek -u cmake gzip libtiff-devel make tesseract-devel
+RUN --mount=type=cache,id=poldek,target=/var/cache/poldek \
+	poldek -O 'keep downloads=yes' --up -u \
+	cmake gzip libtiff-devel make tesseract-devel
 
 FROM build-$TARGET AS build
 WORKDIR /build