appliance/ubuntu: clarify state of ubuntu container

see also #3

appliance/ubuntu/.dockignore

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+tmp*
+.tmp
+.sw?
+README.*

appliance/ubuntu/README.md

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+Note: we currently focus on the Alpine container, because
+
+- Alpine is frequently used in docker due to the small footprint
+- We have a firm grip on Alpine packaging and can quickly build
+  ourselfs all components that we need (quick turn-around cycle)
+- the appliance container is not yet mature, so duplicating work
+  sounds counter-productive
+
+Once the Alpine container is really production grade and if there is
+demand for others, we plan to create appliances with other base OS.
+Ubuntu and CentOS would be choices.
+
+see also: https://github.com/rsyslog/rsyslog-docker/issues/3