[Docker] Include rust in builder stage

The python cryptography library isn't able to be built without the rust
compiler for some stages the cross-platform buildx workflow. This hasn't
been the case in the past, but for some reason since upgrading to the
latest cryptography version there are now errors with installing that
package. Since dependencies in the builder stage are not included in the
final build, we can safely include rust as a fallback to ensure this
doesn't happen without bloating the size of the final build.

Dockerfile

@@ -5,7 +5,8 @@ RUN apk --update add \
     libxml2-dev \
     libxslt-dev \
     openssl-dev \
-    libffi-dev
+    libffi-dev \
+    rust
 
 COPY requirements.txt .