make: use Docker named volumes for ~21x faster local linting

This commit optimizes Docker cache mounting for the linter with a
CI-aware strategy:

**Local development (macOS/Windows)**: Uses Docker named volumes which
keep data inside Docker's native Linux filesystem, avoiding the slow
host-syncing overhead of bind mounts. This yields ~21x faster linting
on warm cache.

**CI (GitHub Actions)**: Uses bind mounts to host paths (`~/.cache/go-build`,
`~/go/pkg/mod`) that GitHub Actions already caches via the setup-go
action. This ensures CI benefits from cached dependencies across runs.

The Makefile detects CI mode via the `CI` environment variable that
GitHub Actions sets automatically.

Local benchmark results:
- Cold run (empty cache): ~2m 28s
- Warm run (cached): ~11s (~21x faster)

Key improvements in warm runs:
- Go packages loading: 1m 58s → 5.6s
- Linters execution: 20.5s → 2.7s
- Total execution: 2m 20s → 8.6s

Author: Roasbeef

Makefile

@@ -68,12 +68,28 @@ ifneq ($(workers),)
 LINT_WORKERS = --concurrency=$(workers)
 endif
 
+# Docker cache mounting strategy:
+# - CI (GitHub Actions): Use bind mounts to host paths that GA caches persist.
+# - Local: Use Docker named volumes (much faster on macOS/Windows due to
+#   avoiding slow host-syncing overhead).
+# Paths inside container must match GOCACHE/GOMODCACHE in tools/Dockerfile.
+ifdef CI
+# CI mode: bind mount to host paths that GitHub Actions caches.
 DOCKER_TOOLS = docker run \
   --rm \
-  -v $(shell bash -c "$(GOCC) env GOCACHE || (mkdir -p /tmp/go-cache; echo /tmp/go-cache)"):/tmp/build/.cache \
-  -v $(shell bash -c "$(GOCC) env GOMODCACHE || (mkdir -p /tmp/go-modcache; echo /tmp/go-modcache)"):/tmp/build/.modcache \
-  -v $(shell bash -c "mkdir -p /tmp/go-lint-cache; echo /tmp/go-lint-cache"):/root/.cache/golangci-lint \
+  -v $${HOME}/.cache/go-build:/tmp/build/.cache \
+  -v $${HOME}/go/pkg/mod:/tmp/build/.modcache \
+  -v $${HOME}/.cache/golangci-lint:/root/.cache/golangci-lint \
   -v $$(pwd):/build lnd-tools
+else
+# Local mode: Docker named volumes for fast macOS/Windows performance.
+DOCKER_TOOLS = docker run \
+  --rm \
+  -v lnd-go-build-cache:/tmp/build/.cache \
+  -v lnd-go-mod-cache:/tmp/build/.modcache \
+  -v lnd-go-lint-cache:/root/.cache/golangci-lint \
+  -v $$(pwd):/build lnd-tools
+endif
 
 GREEN := "\\033[0;32m"
 NC := "\\033[0m"
@@ -472,6 +488,11 @@ clean-mobile:
 	$(RM) -r mobile/build
 	$(RM) mobile/*_generated.go
 
+#? clean-docker-volumes: Remove Docker cache volumes used for local development
+clean-docker-volumes:
+	@$(call print, "Removing Docker cache volumes.")
+	docker volume rm lnd-go-build-cache lnd-go-mod-cache lnd-go-lint-cache 2>/dev/null || true
+
 .PHONY: all \
 	btcd \
 	default \
@@ -500,4 +521,5 @@ clean-mobile:
 	ios \
 	android \
 	mobile \
-	clean
+	clean \
+	clean-docker-volumes