Docker Sandbox kits solve the 'empty sandbox' problem that makes isolated development environments frustrating to use. A kit is a spec.yaml file that describes what tools to install, which network domains to allow, and how to wire credentials securely through a proxy without exposing real secrets inside the microVM. Kits come in two shapes: full sandbox definitions and mixins. Mixins are the practical choice — each one adds a single capability (a JDK, gcloud CLI, YouTube transcript tools) and they stack together at runtime. This makes sandbox setup repeatable and shareable via OCI registries, replacing fragile wiki pages and manual credential handling. The result is a disposable environment that doesn't require constant reconstruction, making isolation actually viable for daily developer workflows.
