Install cryptography build dependencies in requirements image.

This is needed because poetry depends on cryptography. Without this,
building fails when building on an architecture where an upstream
cryptography wheel is not available - for example, on an
armv7 (raspberry pi 4 not in 64 bits mode).

changelog.d/13372.docker

@@ -0,0 +1 @@
+Make docker images build on armv7 by installing cryptography dependencies in the "requirements" stage. Contributed by Jasper Spaans.

docker/Dockerfile

@@ -40,7 +40,8 @@ FROM docker.io/python:${PYTHON_VERSION}-slim as requirements
 RUN \
    --mount=type=cache,target=/var/cache/apt,sharing=locked \
    --mount=type=cache,target=/var/lib/apt,sharing=locked \
- apt-get update -qq && apt-get install -yqq git \
+    apt-get update -qq && apt-get install -yqq \
+      build-essential cargo git libffi-dev libssl-dev \
     && rm -rf /var/lib/apt/lists/*
 
 # We install poetry in its own build stage to avoid its dependencies conflicting with