feat: upgrade Dockerfile

Dockerfile

 FROM ruby:2.6
 MAINTAINER Jacek Lebioda "jacek.lebioda@uni.lu"
+RUN gem install bundler:2.0.2 && \
+    gem install sassc -- --disable-march-tune-native
 COPY Gemfile .
 COPY Gemfile.lock .
-RUN gem install bundler:2.0.2 && \
-    gem install sassc -- --disable-march-tune-native && \
-    bundle install && bundle update
+RUN bundle install && bundle update

readme.md

@@ -4,7 +4,8 @@ A Docker base-image containing Ruby dependencies for Jekyll instances using Jeky
 # Versions
 |   Tag             | Jekyll version | Bundler version | Theme version |
 |:-----------------:|:--------------:|-----------------|---------------|
-| 1.2 (or latest)   |   Jekyll 4.0.0 | 2.0.2           | 0.3.3         |
+| 1.3 (or latest)   |   Jekyll 4.0.0 | 2.0.2           | 0.3.3         |
+| 1.2               |   Jekyll 4.0.0 | 2.0.2           | 0.3.3         |
 | 1.1               |   Jekyll 4.0.0 | 2.0.2           | 0.3.3         |
 | 1.0               |   Jekyll 4.0.0 | 2.0.2           | 0.3.0         |
 | 0.1 (or legacy)   |   Jekyll 3.8.6 | 1.17.3          | 0.3.0         |
@@ -19,8 +20,20 @@ image: git-r3lab.uni.lu:4567/r3/docker/jekyll-lcsb:latest
 image: git-r3lab.uni.lu:4567/r3/docker/jekyll-lcsb:0.1
 ```
 
+For the reproducibility, it's good if you pin the version to a specific tag, e.g. `1.3` instead of `latest`.
+
 # How to build and push to the repository?
-Run:
+It is handled by gitlab-CI automatically (see `.gitlab-ci.yml`) when you tag the commit.
+
+Anyway, if you'd like to do it yourself, perform these steps:
+
+Login to gitlab registry (you can generate the token to login with https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html).
+
+```
+docker login git-r3lab.uni.lu:4567
+```
+
+Then, build the image:
 
 ```
 # replace `new_tag` with e.g. 1.3, and don't forget the dot...