Expand more Makefile variables to support env vars

Makefile

@@ -1,15 +1,15 @@
 .PHONY: all serverless deps docker docker-cgo clean docs test test-race test-integration fmt lint install deploy-docs
 
-TAGS =
+TAGS ?=
 
-INSTALL_DIR = $(GOPATH)/bin
-WEBSITE_DIR = ./website
-DEST_DIR = ./target
+INSTALL_DIR ?= $(GOPATH)/bin
+WEBSITE_DIR ?= ./website
+DEST_DIR ?= ./target
 PATHINSTBIN = $(DEST_DIR)/bin
 PATHINSTTOOLS = $(DEST_DIR)/tools
 PATHINSTSERVERLESS = $(DEST_DIR)/serverless
 PATHINSTDOCKER = $(DEST_DIR)/docker
-DOCKER_IMAGE   ?= jeffail/benthos
+DOCKER_IMAGE   ?= jeffail/benthos
 
 VERSION := $(shell git describe --tags || echo "v0.0.0")
 VER_CUT := $(shell echo $(VERSION) | cut -c2-)
@@ -22,9 +22,9 @@ DATE := $(shell date +"%Y-%m-%dT%H:%M:%SZ")
 VER_FLAGS = -X github.com/benthosdev/benthos/v4/internal/cli.Version=$(VERSION) \
  -X github.com/benthosdev/benthos/v4/internal/cli.DateBuilt=$(DATE)
 
-LD_FLAGS = -w -s
-GO_FLAGS =
-DOCS_FLAGS =
+LD_FLAGS ?= -w -s
+GO_FLAGS ?=
+DOCS_FLAGS ?=
 
 APPS = benthos
 all: $(APPS)