[VOL-5063] - Fix stale voltctl binary install by docker.

docker/Dockerfile.voltctl
-------------------------
  o voltctl v1.6.1 installed by hardcoded path.
    - v1.8.3 released, v1.9.1 latest available (release pending).
  o Download into a temp file to avoid contention or access problems.
  o Split download and install into two distinct steps:
    - Helps create a tiny window for installation.
    - Binary corrupt/unusable while downloading.
  o Plenty of comments added.
  o Long term revisit this logic, branch==master should retrieve/install
    latest tagged release binary.  Release cycle needs to retrieve a
    frozen binary but only on the release branch/tag.

Change-Id: I6f301b934b67dcd3061c7b48386d016ab62e8a82
diff --git a/Makefile b/Makefile
index 75f58a1..24d6433 100644
--- a/Makefile
+++ b/Makefile
@@ -856,8 +856,21 @@
 voltctl-docker-image-build:
 	cd docker && docker build -t opencord/voltctl:local -f Dockerfile.voltctl .
 
+## -----------------------------------------------------------------------
+## -----------------------------------------------------------------------
 voltctl-docker-image-install-kind:
 	@if [ "`kind get clusters | grep kind`" = '' ]; then echo "no kind cluster found" && exit 1; fi
 	kind load docker-image --name `kind get clusters | grep kind` opencord/voltctl:local
 
+## -----------------------------------------------------------------------
+## Intent: [yuck] replace with a standalone reusable installer 4script
+## -----------------------------------------------------------------------
+.PHONY: voltctl-download-url
+voltctl-download-url:
+	curl --silent https://api.github.com/repos/opencord/voltctl/releases/latest \
+	    | grep 'browser_download_url' \
+	    | grep 'linux-amd64'
+
+# [latest] https://github.com/opencord/voltctl/releases/download/untagged-cd611c39178f25b95a87/voltctl-1.9.1-linux-amd64
+
 # [EOF]