Bases

Every rock is built from a base, which defines the baseline system that the rock’s contents are layered on. The systems are Ubuntu releases. An empty, or bare base, is also available for lean images.

The base is declared in the project file by the base key.

Ubuntu bases

When building with an Ubuntu base, Rockcraft pulls the corresponding Ubuntu image from a container registry. These Ubuntu images are the same as the ones hosted in registries like Docker Hub or Amazon Elastic Container Registry.

The built rock contains the software and utilities from the Ubuntu image.

Setting an Ubuntu base for a rock is especially useful when the goal is to build a rock that can serve as a general-purpose environment, such as a development workspace.

LTS and interim bases

Ubuntu bases are divided into LTS and interim bases. An LTS base contains an Ubuntu LTS release and has a 10-year support window. An interim base contains an interim Ubuntu release and is supported for nine months.

Interim bases have shorter lifespans and contain upcoming or experimental features. For rocks, they are most suitable for testing software with short lifecycles. Commonly, images with these bases are used as test-beds for the software in the Ubuntu release itself.

Bare bases

As the name suggests, a bare base results in a rock with no baseline system. This definition is similar to the scratch Docker image, with the exception that a rock can never be completely empty, as it must always include a Pebble and some additional metadata.

The bare base is especially useful when the goal is to build a lean chiselled rock. Such rocks are typically preferred for production environments due to their optimized deployment efficiency and reduced attack surface. The combination of the bare base with Chisel slices will result in a minimalist container image that meets the production environment’s criteria while retaining its functionality.

Even with a bare base, when Rockcraft assembles a rock, it needs Ubuntu as the operating system for its build environment. The project’s build-base key determines which Ubuntu release is provided for the build environment.

For example, if the goal is to have a tiny chiselled rock with software components coming from the Ubuntu 24.04 release, then the project file must have base: bare and build-base: ubuntu@24.04.

How to chisel an existing rock provides a practical example on how to start from a rock with an Ubuntu base and apply the changes necessary to build a chiselled rock with a bare base.