Merge branch 'add-docker-base-image' into 'master'

Add docker base image

See merge request common/dockerfiles!35

base/docker:20.10.5

+FROM docker:20.10.5
+
+MAINTAINER Tamas Gal <tgal@km3net.de>
+
+# for 'egrep -R' and 'xargs -d'
+RUN apk add grep findutils