Add rustc to Docker image build environment (#9405)

This is needed to build the cryptography library, since it does not
provide wheels for ARMv7.

Fixes #9403

Signed-off-by: Dan Callahan <danc@element.io>

changelog.d/9405.misc

+Add rustc as a dependency when building Docker images.

docker/Dockerfile

@@ -28,11 +28,13 @@ RUN apt-get update && apt-get install -y \
     libwebp-dev \
     libxml++2.6-dev \
     libxslt1-dev \
+    rustc \
     zlib1g-dev \
  && rm -rf /var/lib/apt/lists/*
 
 # Build dependencies that are not available as wheels, to speed up rebuilds
 RUN pip install --prefix="/install" --no-warn-script-location \
+        cryptography \
         frozendict \
         jaeger-client \
         opentracing \