Use explicit buildx builder with limited parallelism

.gitignore

@@ -1,3 +1,4 @@
 /Dockerfile
 /ffbuild/
 /artifacts/
+/.cache/

makeimage.sh

@@ -3,14 +3,39 @@ set -xe
 cd "$(dirname "$0")"
 source util/vars.sh
 
+TMPCFG="$(mktemp --suffix=.toml)"
+cat <<EOF >"$TMPCFG"
+[worker.oci]
+  max-parallelism = 4
+EOF
+trap "rm -f '$TMPCFG'" EXIT
+
+docker buildx inspect ffbuilder &>/dev/null || docker buildx create \
+    --bootstrap \
+    --name ffbuilder \
+    --config "$TMPCFG" \
+    --driver-opt env.BUILDKIT_STEP_LOG_MAX_SIZE=-1 \
+    --driver-opt env.BUILDKIT_STEP_LOG_MAX_SPEED=-1
+
 if [[ -z "$QUICKBUILD" ]]; then
     if grep "FROM.*base.*" "images/base-${TARGET}/Dockerfile" >/dev/null 2>&1; then
-        docker buildx --builder default build --load --tag "$BASE_IMAGE" images/base
+        docker buildx --builder ffbuilder build \
+            --cache-from=type=local,src=.cache/"${BASE_IMAGE/:/_}" \
+            --cache-to=type=local,mode=max,dest=.cache/"${BASE_IMAGE/:/_}" \
+            --load --tag "$BASE_IMAGE" images/base
     fi
 
-    docker buildx --builder default build --load --build-arg GH_REPO="$REPO" --tag "$TARGET_IMAGE" "images/base-${TARGET}"
+    docker buildx --builder ffbuilder build \
+        --cache-from=type=local,src=.cache/"${TARGET_IMAGE/:/_}" \
+        --cache-to=type=local,mode=max,dest=.cache/"${TARGET_IMAGE/:/_}" \
+        --load --build-arg GH_REPO="$REPO" --tag "$TARGET_IMAGE" "images/base-${TARGET}"
 fi
 
 ./generate.sh "$TARGET" "$VARIANT" "${ADDINS[@]}"
 
-docker buildx --builder default build --load --tag "$IMAGE" .
+docker buildx --builder ffbuilder build \
+    --cache-from=type=local,src=.cache/"${IMAGE/:/_}" \
+    --cache-to=type=local,mode=max,dest=.cache/"${IMAGE/:/_}" \
+    --load --tag "$IMAGE" .
+
+docker buildx rm -f ffbuilder