Makefile: Use 'go install ...' for the install target

GOBIN (documented in [1]) defaults to DIR/bin, but it's a configurable
variable in its own right.  The old logic was just looking at GOPATH,
though, and not respecting GOBIN.

This commit updates our install target to lean on Go's build caching
(instead of using Make's dependency trees) to ensure a fresh-enough
build lands in the appropriate directory.  This approach relies on Go
1.10+ to avoid [2], but we've required Go 1.10+ since b859ebf
(Documentation/development: Bump minimum Go version to 1.10,
2018-04-03, #955).

For the build documentation, I've switched to 'go env GOPATH' to get
the (platform-specific [1]) default value when the environment
variable is not set.  And I've used cut [3] (instead of the awk [4]
the Makefile used to use) to pull out the first component of GOPATH.
Both are in POSIX, but cut is a simpler tool for this particular
problem.

[1]: https://golang.org/cmd/go/#hdr-GOPATH_environment_variable
[2]: golang/go#18981
[3]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/cut.html
[4]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/awk.html

Author: wking

Documentation/development.md

@@ -3,15 +3,14 @@
 ## Requirements
 
 * Go 1.10+
-* Configured [GOPATH](http://golang.org/doc/code.html#GOPATH)
 
 ## Building
 
-First, clone the repo into the proper location in your $GOPATH:
+First, clone the repo into the proper location in your [`GOPATH`][GOPATH]:
 
 ```
 go get -u github.com/kubernetes-incubator/bootkube
-cd $GOPATH/src/github.com/kubernetes-incubator/bootkube
+cd $(go env GOPATH | cut -d: -f1)/src/github.com/kubernetes-incubator/bootkube
 ```
 
 Then build:
@@ -71,3 +70,5 @@ Commenting on the PR:
 * `coreosbot run e2e checkpointer`: can be used to specifically test new checkpointer code.
     * This will build a new checkpointer image from the PR, and includes that image as part of the checkpointer daemonset.
 * `coreosbot run conformance`: run upstream Kubernetes conformance tests
+
+[GOPATH]: https://golang.org/cmd/go/#hdr-GOPATH_environment_variable

Makefile

@@ -5,7 +5,9 @@ export PATH:=$(PATH):$(PWD)
 SHELL:=$(shell which bash)
 LOCAL_OS:=$(shell uname | tr A-Z a-z)
 GOFILES:=$(shell find . -name '*.go' | grep -v -E '(./vendor)')
-GOPATH_BIN:=$(shell echo ${GOPATH} | awk 'BEGIN { FS = ":" }; { print $1 }')/bin
+GOPATH ?= $(shell go env GOPATH)
+PRIMARY_GOPATH ?= $(shell echo ${GOPATH} | cut -d : -f 1)
+GOBIN ?= $(PRIMARY_GOPATH)/bin
 LDFLAGS=-X github.com/kubernetes-incubator/bootkube/pkg/version.Version=$(shell $(CURDIR)/build/git-version.sh)
 TERRAFORM:=$(shell command -v terraform 2> /dev/null)
 
@@ -41,7 +43,7 @@ endif
 	@go test -v $(shell go list ./... | grep -v '/vendor/\|/e2e')
 
 install: _output/bin/$(LOCAL_OS)/bootkube
-	cp $< $(GOPATH_BIN)
+	cp $< $(GOBIN)
 
 _output/bin/%: GOOS=$(word 1, $(subst /, ,$*))
 _output/bin/%: GOARCH=$(word 2, $(subst /, ,$*))