SW-612: Specify Python3 executable to satisfy cmake

Docker/lofar-pipeline/Dockerfile.tmpl

@@ -91,7 +91,7 @@ RUN apt-get update && apt-get install -y subversion cmake g++ gfortran bison fle
     cd ${INSTALLDIR}/lofar && \
     svn --non-interactive -q co -r ${LOFAR_REVISION} -N ${LOFAR_BRANCH_URL} src; \
     svn --non-interactive -q up src/CMake && \
-    cd ${INSTALLDIR}/lofar/build/${LOFAR_BUILDVARIANT} && cmake -DBUILD_PACKAGES=Offline -DBUILD_TESTING=OFF -DCMAKE_INSTALL_PREFIX=${INSTALLDIR}/lofar/ -DCASAREST_ROOT_DIR=${INSTALLDIR}/casarest/ -DCASACORE_ROOT_DIR=${INSTALLDIR}/casacore/ -DAOFLAGGER_ROOT_DIR=${INSTALLDIR}/aoflagger/ -DBDSF_ROOT_DIR=/opt/pybdsf/lib/python${PYTHON_VERSION}/site-packages/ -DQPID_ROOT_DIR=/opt/qpid/ -DUSE_OPENMP=True ${INSTALLDIR}/lofar/src/ && \
+    cd ${INSTALLDIR}/lofar/build/${LOFAR_BUILDVARIANT} && cmake -DBUILD_PACKAGES=Offline -DBUILD_TESTING=OFF -DCMAKE_INSTALL_PREFIX=${INSTALLDIR}/lofar/ -DPYTHON_EXECUTABLE=/usr/bin/python3 -DCASAREST_ROOT_DIR=${INSTALLDIR}/casarest/ -DCASACORE_ROOT_DIR=${INSTALLDIR}/casacore/ -DAOFLAGGER_ROOT_DIR=${INSTALLDIR}/aoflagger/ -DBDSF_ROOT_DIR=/opt/pybdsf/lib/python${PYTHON_VERSION}/site-packages/ -DQPID_ROOT_DIR=/opt/qpid/ -DUSE_OPENMP=True ${INSTALLDIR}/lofar/src/ && \
     cd ${INSTALLDIR}/lofar/build/${LOFAR_BUILDVARIANT} && sed -i '29,31d' include/ApplCommon/PosixTime.h && \
     cd ${INSTALLDIR}/lofar/build/${LOFAR_BUILDVARIANT} && make -j ${J} && \
     cd ${INSTALLDIR}/lofar/build/${LOFAR_BUILDVARIANT} && make install && \