The ongoing revolution in user-defined software stacks (UDSS) via containerization, virtualization and other methods has dramatically changed how applications and services are delivered across the entire computing industry. Popularized by Docker, this shift has established a new ecosystem of tools and techniques with new, more flexible and agile approaches, thereby also gaining traction in the HPC community. In addition to HPC-optimized container runtimes, emerging technologies like Kubernetes and Podman create a new set of opportunities and challenges. While adoption is growing, there remain numerous questions regarding best practices, foundational concepts, tools and standards. Our goal is to promote and accelerate the adoption and impact of this new ecosystem to better address HPC use cases. This workshop serves as a key venue for presenting late-breaking research, sharing experiences and best practices and fostering collaboration in this field. Our second workshop iteration will emphasize real-world experiences and challenges in HPC UDSS.