Add data containers to fix permission issues

data/Dockerfiles/data-container-dkim/Dockerfile

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+FROM debian:jessie
+
+RUN mkdir -p /data/dkim/keys \
+	&& mkdir -p /data/dkim/txt \
+	&& chown -R 33:33 /data/dkim
+VOLUME /data/dkim
+USER www-data
+CMD ["echo", "Data container for dkim keys"]

data/Dockerfiles/data-container-vmail/Dockerfile

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+FROM debian:jessie
+
+RUN groupadd -g 5000 vmail \
+	&& useradd -g vmail -u 5000 vmail -d /var/vmail
+
+RUN mkdir -p /var/vmail \
+  && chown -R 5000:5000 /var/vmail
+VOLUME /var/vmail
+USER vmail
+CMD ["echo", "Data container for mail"]