Re #93 - tweak build packages

Dockerfile

@@ -5,8 +5,14 @@ FROM python:3.8-slim as builder
 ARG CRYPTOGRAPHY_DONT_BUILD_RUST=1
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    libssl-dev
-
+    libssl-dev \
+    libffi-dev \
+    gcc \
+    libc-dev \
+    libxslt-dev \
+    zlib1g-dev \
+    g++
+    
 RUN mkdir /install
 WORKDIR /install