Gitlab CI: improved workflow

Runner now creates test tags for non-master branches.  For the master
branch, the image is assigned the "latest" tag.

.gitlab-ci.yml

@@ -3,4 +3,12 @@ before_script:
 
 build_image:
   script:
-    - docker build -t argonneeic/base:testing --build-arg BUILD_THREADS=20 .
\ No newline at end of file
+    - docker build -t argonneeic/base:$CI_COMMIT_REF_NAME --build-arg BUILD_THREADS=20 .
+  only:
+    - branches
+
+deploy_latest:
+  script:
+    - docker build -t argonneeic/base --build-arg BUILD_THREADS=20 .
+  only:
+    - master