Add CI-based build of rtmgr using multiple stages

Created single Dockerfile based on existing files.
Added tag-definition file that is read by the Jenkins job
that builds a Docker image.
Upgrade to go version 1.12.
Un-ignore and add .gitreview file.

Signed-off-by: Lott, Christopher (cl778h) <cl778h@att.com>
Change-Id: Ic1c15844a83b36e59bd94f8ea5c424e41177908d
diff --git a/.gitreview b/.gitreview
new file mode 100644
index 0000000..a7f2fae
--- /dev/null
+++ b/.gitreview
@@ -0,0 +1,5 @@
+[gerrit]
+host=gerrit.o-ran-sc.org
+port=29418
+project=ric-plt/rtmgr
+defaultbranch=master