BuildKit's default GC caps the ephemeral cache tier (RUN
--mount=type=cache mounts, local context, git checkouts) at a hardcoded
512 MB.
Across the 7-distro build matrix that fills instantly and forces
re-downloads of system and R packages on every rebuild.
Add docker/buildkitd.toml and pass it via --config when creating the
docker-container builders (artemis/amd64, gaia/arm64):
- cache-mount tier raised 512 MB -> 8 GB, retained 7 days
- total cache bounded at 40 GB with 20 GB min-free (safe on the
disk-tight Hetzner host, modest on the Mac mini)
Limits are absolute rather than percentages because the two hosts differ
~6x in free space. Document the --config flag in the justfile so a future
recreate does not silently revert to the 512 MB default.
