Merge branch 'hotfix-docker-wrong-build-dune-env-image' into 'master'

Hotfix: Docker with wrong build dune-env image

See merge request !160

docker/dorie.dockerfile

 ARG BASE_IMG_VERSION=1.2
+ARG DUNE_ENV_IMAGE=dorie/dune-env:img-v${BASE_IMG_VERSION}
 
 # start build image
-FROM dorie/dune-env:img-v${BASE_IMG_VERSION} as build-env
+FROM ${DUNE_ENV_IMAGE} as build-env
 
 # maintainer info
 LABEL maintainer="lriedel@iup.uni-heidelberg.de"