refactor deprecated matrix

.drone.yml

@@ -1,12 +1,39 @@
 pipeline:
 
-  docker:
+  jekyll-3.7:
+    image: plugins/docker
+    repo: bunix42/jekyll
+    build_args:
+      - JEKYLL_VERSION=3.7.2
+    tags:
+      - 3.7.2
+    secrets: [ docker_username, docker_password ]
+
+  jekyll-3.8:
     image: plugins/docker
     repo: bunix42/jekyll
     build_args:
       - JEKYLL_VERSION=3.8.6
     tags:
-      - ${JEKYLL_VERSION}
+      - 3.8.6
+    secrets: [ docker_username, docker_password ]
+
+  jekyll-3.9:
+    image: plugins/docker
+    repo: bunix42/jekyll
+    build_args:
+      - JEKYLL_VERSION=3.9.1
+    tags:
+      - 3.9.1
+    secrets: [ docker_username, docker_password ]
+
+  jekyll-4.2:
+    image: plugins/docker
+    repo: bunix42/jekyll
+    build_args:
+      - JEKYLL_VERSION=4.2.0
+    tags:
+      - 4.2.0
     secrets: [ docker_username, docker_password ]
 
   slack:
@@ -14,10 +41,3 @@ pipeline:
     channel: drone
     username: drone
     secrets: [ slack_webhook ]
-
-matrix:
-  JEKYLL_VERSION:
-    - 3.7.2
-    - 3.8.6
-    - 3.9.1
-    - 4.2.0