Docker: check out packaging tags.

This will ensure we're checking out source code that is close to what we
have in binary packages.

While at it, remove the checkout directory when it's no longer needed.

pkg/docker/Makefile

@@ -6,6 +6,7 @@ include ../shasum.mak
 DEFAULT_VERSION := $(NXT_VERSION)
 
 VERSION ?= $(DEFAULT_VERSION)
+PATCHLEVEL ?= 1
 
 EXPORT_DIR := $(VERSION)
 
@@ -73,6 +74,7 @@ Dockerfile.%: ../../version template.Dockerfile
 	@echo "===> Building $@"
 	cat template.Dockerfile | sed \
 			-e 's,@@VERSION@@,$(VERSION),g' \
+			-e 's,@@PATCHLEVEL@@,$(PATCHLEVEL),g' \
 			-e 's,@@CONTAINER@@,$(CONTAINER_$(call modname, $*)),g' \
 			-e 's,@@CONFIGURE@@,$(CONFIGURE_$(call modname, $*)),g' \
 			-e 's,@@INSTALL@@,$(INSTALL_$(call modname, $*)),g' \