feat: add dnscontrol image

dnscontrol/Dockerfile

+FROM stackexchange/dnscontrol:v3.20.0
+
+LABEL \
+        org.opencontainers.image.title="dnscontrol" \
+        org.opencontainers.image.url="https://code.fbi.h-da.de/hdacloud/images" \
+        org.opencontainers.image.source="https://code.fbi.h-da.de/hdacloud/images"
+
+RUN apk add py3-pip
+RUN pip install python-gitlab
+