Why does apt run out of space inside a Docker build when my Mac has plenty available?

The `df` output from macOS describes the host filesystem, not the Linux filesystem used by Docker Desktop. Containers and BuildKit run inside Docker Desktop’s Linux VM, which has its own virtual disk and filesystem limits. Check the filesystem from inside a temporary container or build stage with commands such as `df -h` and `df -i`, and inspect Docker Desktop’s disk image/storage settings rather than relying on the Mac’s available space.

If the VM or its virtual disk is genuinely full, increase Docker Desktop’s allocated disk space or recreate/resize the Docker disk image. Also check inode usage with `df -i`, since “No space left on device” can mean either blocks or inodes.

The Dockerfile can be made less wasteful, although that alone should not be necessary for a normal 20 GB VM. Avoid `apt upgrade` in an image unless you specifically need it, because it downloads and unpacks many additional packages. Combine the required apt work into one layer and clean it in the same RUN instruction, for example:

`apt-get update && apt-get install -y --no-install-recommends ... && rm -rf /var/lib/apt/lists/* /var/cache/apt/archives/*`

Use `apt-get` rather than `apt` in noninteractive Docker builds. Also note that the package list contains `zlib1g-dev` twice. These changes reduce temporary and final layer size, but if apt cannot even create its lock directory, the primary issue is still the filesystem or inode quota inside the Docker Linux VM.

BuildKit’s garbage-collection setting is not the same thing as the maximum size of the Docker Desktop VM. `docker buildx du` only reports the cache visible to that builder, while the VM can also contain images, active containers, temporary build data, and data from another platform builder. Multi-platform builds may use separate intermediate layers for each architecture.

Verify which builder is active with `docker buildx ls`, inspect it with `docker buildx inspect --bootstrap`, and check Docker Desktop’s VM/disk usage. Make sure you are pruning or removing the builder that is actually performing the build. A 20 GB `defaultKeepStorage` value is a cache policy, not a guarantee that every part of the build environment has 20 GB available.