Dockerfile.base-spack: check if $SPACK_CONFIG_DIR exists (#144)

This change will generate an ENOENT error, if the spack-config directory 
for a particular Spack version does not exist.

Author: harshula

containers/Dockerfile.base-spack

@@ -88,7 +88,9 @@ RUN git clone  https://github.com/ACCESS-NRI/spack-packages.git ${SPACK_PACKAGES
 # Install ACCESS-NRI's spack-config repo
 RUN git clone  https://github.com/ACCESS-NRI/spack-config.git ${SPACK_CONFIG_REPO_ROOT} --branch ${SPACK_CONFIG_REPO_VERSION}
 
-RUN ln -s -r -v ${SPACK_CONFIG_DIR}/* ${SPACK_ROOT}/etc/spack/
+# Exit with ENOENT 2 "No such file or directory", if the directory does not
+# exist
+RUN if [ -d "${SPACK_CONFIG_DIR}" ]; then ln -s -r -v ${SPACK_CONFIG_DIR}/* ${SPACK_ROOT}/etc/spack/; else echo "${SPACK_CONFIG_DIR} does not exist!"; exit 2; fi
 
 # Enables setting Spack setup type via SHELL command
 # docker-shell:      Use for build RUN steps