add matrix build for master and release tag 14.4

.drone.yml

@@ -1,8 +1,18 @@
 pipeline:
 
+  build:
+    image: alpine:edge
+    commands:
+      - sed -i 's@<TAG>@${BUILD_VERSION}@' Dockerfile
+
   docker:
     image: plugins/docker
     repo: bunix42/ttrss
     tags:
-     - latest
+     - ${BUILD_VERSION}
-    secrets: [ docker_username, docker_password ]
+    secrets: [ docker_username, docker_password ]
+    
+matrix:
+  BUILD_VERSION:
+    - master
+    - 14.4