[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/VERSION b/VERSION
index ae93586..aab16c0 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.12.7
+2.12.7.1-dev