ci: use locally built gem file for testing Docker image build

docker/Dockerfile

@@ -8,6 +8,7 @@ RUN apk --no-cache add openssl ca-certificates && \
     apk --no-cache add ruby ruby-etc ruby-io-console ruby-json ruby-webrick && \
     apk --no-cache add --virtual .build-deps linux-headers ruby-dev build-base tzdata && \
     gem install --no-document dyndnsd -v ${DYNDNSD_VERSION} && \
+    rm -rf /usr/lib/ruby/gems/*/cache/ && \
     # set timezone to Berlin
     cp /usr/share/zoneinfo/Europe/Berlin /etc/localtime && \
     apk del .build-deps