update/cleanup dockerfile and parametrize build

.drone.yml

@@ -1,9 +1,19 @@
 pipeline:
 
+  build:
+    image: alpine:edge
+    commands:
+      - sed -i 's@<VERSION>@${VERSION}@' Dockerfile
+
   docker:
     image: plugins/docker
     repo: bunix42/jekyll
     tags:
-     - ${DRONE_REPO_BRANCH}
-    email: hanjo@bunix.de
+     - ${VERSION}
     secrets: [ docker_username, docker_password ]
+
+matrix:
+  VERSION:
+    - 3.5.0
+    - 3.5.2
+    - 3.6.0

Dockerfile

@@ -4,16 +4,14 @@ MAINTAINER Hanjo Meinhardt <hanjo@bunix.de>
 
 ENV dev_packages="ca-certificates build-base libffi-dev ruby-dev ruby-rdoc ruby-irb"
 
-RUN apk update && apk upgrade
+RUN apk update && apk upgrade \
+    && apk add ruby ruby-json ${dev_packages} \
+    && rm -rf /var/cache/apk/*
 
-RUN apk add ruby ruby-json ${dev_packages}
-  
-RUN rm -rf /var/cache/apk/*
-
-RUN gem install jekyll
-RUN gem install jekyll-paginate
-RUN gem install jekyll-auto-image
-RUN gem install jekyll-tidy
-RUN gem install jekyll-sitemap
+RUN gem install jekyll -v <VERSION> \
+    && gem install jekyll-paginate \
+    && gem install jekyll-auto-image \
+    && gem install jekyll-tidy \
+    && gem install jekyll-sitemap
 
 RUN apk del ${dev_packages}