Add some minor reorganization to the Makefile preamble

The gist here is a reemphasizing of the explicitly "user mutable" bits by putting them first (and hopefully improving readability a little bit in the process).

Signed-off-by: Andrew Page <admwiggin@gmail.com>

Makefile

@@ -1,23 +1,39 @@
 .PHONY: all binary build cross default docs docs-build docs-shell shell test test-unit test-integration test-integration-cli validate
 
+# env vars passed through directly to Docker's build scripts
+# to allow things like `make DOCKER_CLIENTONLY=1 binary` easily
+# `docs/sources/contributing/devenvironment.md ` and `project/PACKAGERS.md` have some limited documentation of some of these
+DOCKER_ENVS := \
+	-e BUILDFLAGS \
+	-e DOCKER_CLIENTONLY \
+	-e DOCKER_EXECDRIVER \
+	-e DOCKER_GRAPHDRIVER \
+	-e TESTDIRS \
+	-e TESTFLAGS \
+	-e TIMEOUT
+# note: we _cannot_ add "-e DOCKER_BUILDTAGS" here because even if it's unset in the shell, that would shadow the "ENV DOCKER_BUILDTAGS" set in our Dockerfile, which is very important for our official builds
+
 # to allow `make BINDDIR=. shell` or `make BINDDIR= test`
 # (default to no bind mount if DOCKER_HOST is set)
 BINDDIR := $(if $(DOCKER_HOST),,bundles)
+DOCKER_MOUNT := $(if $(BINDDIR),-v "$(CURDIR)/$(BINDDIR):/go/src/github.com/docker/docker/$(BINDDIR)")
+
+# to allow `make DOCSDIR=docs docs-shell` (to create a bind mount in docs)
+DOCS_MOUNT := $(if $(DOCSDIR),-v $(CURDIR)/$(DOCSDIR):/$(DOCSDIR))
+
 # to allow `make DOCSPORT=9000 docs`
 DOCSPORT := 8000
 
 GIT_BRANCH := $(shell git rev-parse --abbrev-ref HEAD 2>/dev/null)
-GITCOMMIT := $(shell git rev-parse --short HEAD 2>/dev/null)
 DOCKER_IMAGE := docker$(if $(GIT_BRANCH),:$(GIT_BRANCH))
 DOCKER_DOCS_IMAGE := docker-docs$(if $(GIT_BRANCH),:$(GIT_BRANCH))
-DOCKER_MOUNT := $(if $(BINDDIR),-v "$(CURDIR)/$(BINDDIR):/go/src/github.com/docker/docker/$(BINDDIR)")
 
-DOCKER_ENVS := -e TIMEOUT -e BUILDFLAGS -e TESTFLAGS \
-  -e TESTDIRS -e DOCKER_GRAPHDRIVER -e DOCKER_EXECDRIVER \
-  -e DOCKER_CLIENTONLY
 DOCKER_RUN_DOCKER := docker run --rm -it --privileged $(DOCKER_ENVS) $(DOCKER_MOUNT) "$(DOCKER_IMAGE)"
-# to allow `make DOCSDIR=docs docs-shell`
-DOCKER_RUN_DOCS := docker run --rm -it $(if $(DOCSDIR),-v $(CURDIR)/$(DOCSDIR):/$(DOCSDIR)) -e AWS_S3_BUCKET
+
+DOCKER_RUN_DOCS := docker run --rm -it $(DOCS_MOUNT) -e AWS_S3_BUCKET
+
+# for some docs workarounds (see below in "docs-build" target)
+GITCOMMIT := $(shell git rev-parse --short HEAD 2>/dev/null)
 
 default: binary