Makefile improvements.

- Added image double-tagging, images are now always tagged with :latest.
- Version control DIRTY flag is:
  - a) Injected into containers properly on build.
  - b) Used to append -dirty to the image tag. (:latest is unaffected.)
- Re-added DOCKER_BUILD_ARGS to deduplicate code (not sure why this was changed).

Change-Id: I12a40e0e718abcdaedbbf85622856d3d4104d05d
diff --git a/docker/Dockerfile.afrouter b/docker/Dockerfile.afrouter
index de2646f..e604e2c 100644
--- a/docker/Dockerfile.afrouter
+++ b/docker/Dockerfile.afrouter
@@ -64,6 +64,7 @@
 ARG org_label_schema_vcs_ref=unknown
 ARG org_label_schema_build_date=unknown
 ARG org_opencord_vcs_commit_date=unknown
+ARG org_opencord_vcs_dirty=unknown
 
 LABEL org.label-schema.schema-version=1.0 \
       org.label-schema.name=afrouter \
@@ -71,4 +72,5 @@
       org.label-schema.vcs-url=$org_label_schema_vcs_url \
       org.label-schema.vcs-ref=$org_label_schema_vcs_ref \
       org.label-schema.build-date=$org_label_schema_build_date \
-      org.opencord.vcs-commit-date=$org_opencord_vcs_commit_date
+      org.opencord.vcs-commit-date=$org_opencord_vcs_commit_date \
+      org.opencord.vcs-commit-dirty=$org_opencord_vcs_dirty