chore(make): go path init, add init.dev target

Following is changed:
  - Initialize GOPATH for easier execution of the `api-docs` target
  - Add `init.dev` target for installing dev dependencies.

Makefile

@@ -18,13 +18,15 @@
 
 GOCMD := go
 ifdef GOROOT
-GOCMD = $(GOROOT)/bin/go
+	GOCMD = $(GOROOT)/bin/go
 endif
 
-SWAG := swag
-ifdef GOPATH
-SWAG = $(GOPATH)/bin/swag
+ifndef $(GOPATH)
+    GOPATH=$(shell go env GOPATH)
+    export GOPATH
 endif
+
+SWAG = $(GOPATH)/bin/swag
 SWAG_ARGS := -d api/v1/,cmd/tank/ -g api.go
 
 EXECUTEABLE := tank
@@ -32,6 +34,10 @@ EXECUTEABLE := tank
 all: build
 .PHONY: vet format ui build clean distclean
 
+init.dev::
+	go get -u github.com/swaggo/swag/cmd/swag
+	go install github.com/swaggo/swag/cmd/swag@latest
+
 vet:
 	$(GOCMD) vet ./...