Modern Software Stack Building for HEP

High-Energy Physics has evolved a rich set of software packages that need to work harmoniously to carry out the key software tasks needed by experiments. The problem of consistently building and deploying these packages as a coherent software stack is one that is shared across the HEP community. To that end the HEP Software Foundation Packaging Working Group has worked to identify common solutions that can be used across experiments, with an emphasis on consistent, reproducible builds and easy deployment into CernVM-FS or containers via CI systems. We based our approach on well-identified use cases and requirements from many experiments. In this paper we summarise the work of the group in the last year and how we have explored various approaches based on package managers from industry and the scientific computing community. We give details about a solution based on the Spack package manager which has been used to build the software required by the SuperNEMO and FCC experiments and trialled for a multi-experiment software stack, Key4hep. We shall discuss changes that needed to be made to Spack to satisfy all our requirements. We show how support for a build environment for software developers is provided.