Add a Dockerfile that allows using a base image with a cargo cache

docker/Dockerfile

@@ -12,11 +12,13 @@
 #
 
 ARG PYTHON_VERSION=3.8
+ARG BASE_IMAGE=docker.io/python:${PYTHON_VERSION}-slim
+ARG CARGO_NET_OFFLINE=false
 
 ###
 ### Stage 0: builder
 ###
-FROM docker.io/python:${PYTHON_VERSION}-slim as builder
+FROM ${BASE_IMAGE} as builder
 
 # install the OS build deps
 RUN apt-get update && apt-get install -y \
@@ -32,6 +34,8 @@ RUN apt-get update && apt-get install -y \
     zlib1g-dev \
  && rm -rf /var/lib/apt/lists/*
 
+ENV CARGO_NET_OFFLINE=${CARGO_NET_OFFLINE}
+
 # Build dependencies that are not available as wheels, to speed up rebuilds
 RUN pip install --prefix="/install" --no-warn-script-location \
         cryptography \

docker/Dockerfile-cargo-cache

@@ -0,0 +1,21 @@
+# A docker file that caches the cargo index for the cryptography deps. This is
+# mainly useful for multi-arch builds where fetching the index from the internet
+# fails for 32bit archs built on 64 bit platforms.
+
+ARG PYTHON_VERSION=3.8
+
+FROM --platform=$BUILDPLATFORM docker.io/python:${PYTHON_VERSION}-slim as builder
+
+RUN apt-get update && apt-get install -y \
+    rustc \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN pip download --no-binary cryptography --no-deps cryptography
+
+RUN tar -xf cryptography*.tar.gz --wildcards cryptography*/src/rust/
+
+RUN cd cryptography*/src/rust && cargo fetch
+
+FROM docker.io/python:${PYTHON_VERSION}-slim
+
+COPY --from=builder /root/.cargo /root/.cargo