ROHD-2747: Fix use of build variables, and consume superfluous ones supplied by our build setup

Docker/lofar-outputproc/Dockerfile.tmpl

@@ -5,6 +5,9 @@ FROM lofar-base:${LOFAR_TAG}
 
 RUN apt-get update && apt-get install -y git python python3 g++ make
 
+# Consume superfluous build variables
+ENV DOCKER_IMAGE_BUILD_DATE=${DOCKER_IMAGE_BUILD_DATE}
+
 #
 # *******************
 #   Blitz
@@ -55,7 +58,7 @@ RUN apt-get update && apt-get install -y binutils liblog4cplus-1.1-9 libxml2 lib
 
 # Tell image build information
 ENV LOFAR_BRANCH=${LOFAR_VERSION} \
-    LOFAR_BUILDVARIANT=gnucxx11_opt
+    LOFAR_BUILDVARIANT=${LOFAR_BUILDVARIANT}
 
 # Install
 RUN apt-get update && apt-get install -y git cmake g++ gfortran bison flex autogen liblog4cplus-dev libhdf5-dev libboost-dev boost-python${BOOST_VERSION}-dev libxml2-dev pkg-config libpng-dev libfftw3-dev libunittest++-dev libxml++2.6-dev libboost-filesystem${BOOST_VERSION}-dev libboost-date-time${BOOST_VERSION}-dev libboost-thread${BOOST_VERSION}-dev libboost-regex${BOOST_VERSION}-dev binutils-dev libopenblas-dev libcfitsio-dev wcslib-dev libcap2-bin libreadline-dev && \