add dedicated docker container for some utils

fixes https://github.com/zokradonh/kopano-docker/issues/57

Signed-off-by: Felix Bartels <felix@host-consultants.de>

utils/Dockerfile

@@ -0,0 +1,9 @@
+ARG docker_repo=zokradonh
+FROM ${docker_repo}/kopano_core
+
+RUN apt update && apt-get install --no-install-recommends -y \
+    kopano-backup \
+    kopano-migration-imap \
+    kopano-migration-pst
+
+CMD [ "/bin/bash" ]