Update Dockerfile to fix break

This follow MiniFate/MiniFate#123

Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:20.04
+FROM ruby:2
 
 # Avoid prompt for time zone info.
 ENV DEBIAN_FRONTEND=noninteractive
@@ -9,20 +9,12 @@ ENV LC_ALL=C.UTF-8
 ENV LANG=$US_UTF
 ENV LANGUAGE=$US_UTF
 
-# Install requirements for bundle
-RUN apt-get -y update \
-    && apt-get install -y \
-        build-essential \
-        git \
-        ruby \
-        ruby-dev \
-        zlib1g-dev
-
 # Install required gems via bundle
-ADD Gemfile .
-RUN gem install bundler \
-    && gem update --system \
-    && bundle install \
-    && bundle update
+RUN gem install bundler -v 2.4.22
+
+ENV BUNDLE_GEMFILE=/tmp/Gemfile
+COPY Gemfile $BUNDLE_GEMFILE
+
+RUN bundle install -j 4
 
 CMD ["/bin/bash"]