feat(build): raise BuildKit cache-mount budget for remote builders

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.

docker/buildkitd.toml

@@ -0,0 +1,41 @@
+# BuildKit GC config for the remote buildx builders (artemis/amd64, gaia/arm64).
+#
+# Apply when creating the docker-container builders:
+#   docker buildx create --name artemis --driver docker-container \
+#     --config docker/buildkitd.toml ssh://<user@host-amd64>
+#   docker buildx create --name gaia    --driver docker-container \
+#     --config docker/buildkitd.toml ssh://<user@host-arm64>
+#
+# Why: BuildKit's default GC caps the ephemeral cache tier — RUN
+# --mount=type=cache mounts, local build context, git checkouts — at a
+# hardcoded 512 MB (shown as "488.3 MiB" in `buildx inspect`). Across our
+# 7-distro build matrix that fills instantly and forces re-downloads of
+# system + R packages every rebuild. The first rule below raises that tier.
+#
+# Limits are absolute (not %) on purpose: artemis and gaia have very
+# different free space (Hetzner ~42 GiB free vs Mac mini ~279 GiB), so a
+# percentage would mean wildly different real budgets. minFreeSpace = 20 GB
+# keeps the tight Hetzner host safe while staying modest on the Mac mini.
+
+[worker.oci]
+  gc = true
+
+  # Tier 1 — ephemeral caches (cache mounts, local context, git checkouts).
+  # Raised from the 512 MB default to 8 GB, retained for 7 days so weekly
+  # rebuilds reuse downloaded packages instead of re-fetching them.
+  [[worker.oci.gcpolicy]]
+    filters = [
+      "type==source.local",
+      "type==exec.cachemount",
+      "type==source.git.checkout",
+    ]
+    keepDuration = "168h"
+    maxUsedSpace = "8GB"
+
+  # Tier 2 — everything else (image layers, RUN exec results). Bounds the
+  # whole buildkit cache and always leaves 20 GB free on the host disk.
+  [[worker.oci.gcpolicy]]
+    all = true
+    reservedSpace = "2GB"
+    maxUsedSpace = "40GB"
+    minFreeSpace = "20GB"