Dockerfile fails to build

[sattlerc]

(Extracted from here.)

Build log:

Sending build context to Docker daemon  138.7MB
Step 1/9 : FROM ruby:2.3
 ---> c0d23ebb3ed6
Step 2/9 : RUN apt-get update -y && apt-get install -y     rake rubygems ruby-sqlite3 libxslt-dev libxml2-dev libsqlite3-dev swig flex bison     && rm -rf /var/lib/apt/lists/* &&     gem update --system && gem update
 ---> Using cache
 ---> 719e22e205e8
Step 3/9 : RUN mkdir -p /usr/src/app
 ---> Using cache
 ---> 345d7f71afc2
Step 4/9 : WORKDIR /usr/src/app
 ---> Using cache
 ---> 83a1116cda6f
Step 5/9 : COPY Gemfile /usr/src/app/
 ---> 39bfb8d8a993
Step 6/9 : RUN bundle install
 ---> Running in b17dea5a2a7c
Fetching https://github.com/distler/maruku.git
Fetching gem metadata from https://rubygems.org/...........
Resolving dependencies...
Resolving dependencies...
Resolving dependencies...
Resolving dependencies...
Bundler found conflicting requirements for the Ruby version:
  In Gemfile:
    rubyzip (~> 2.3.0) was resolved to 2.3.2, which depends on
      Ruby (>= 2.4)

  Current Ruby version:
    Ruby (= 2.3.8)
The command '/bin/sh -c bundle install' returned a non-zero code: 6

I was able to build after changing ruby:2.3 to ruby:2.6.