Install fails

[joex19x84]

During the bundle install step, I get this error (the log is of no help to me). I've also tried to install via the docker file with no documentation (it fails as well, see second error). Also, what is this RAILS_ENV variable, which is not set, by the way? Why does a docker app depend on Ruby on Rails? This application looks nice, but this installation seems prone to failure.

    --without-mysqlclient-include=${mysqlclient-dir}/include
    --with-mysqlclient-lib
    --without-mysqlclient-lib=${mysqlclient-dir}/lib
    --with-mysqlclientlib
    --without-mysqlclientlib

/home/joe/.rvm/rubies/ruby-2.7.1/lib/ruby/2.7.0/mkmf.rb:1050:in block in find_library': undefined method split' for nil:NilClass
(NoMethodError)
from /home/joe/.rvm/rubies/ruby-2.7.1/lib/ruby/2.7.0/mkmf.rb:1050:in collect' from /home/joe/.rvm/rubies/ruby-2.7.1/lib/ruby/2.7.0/mkmf.rb:1050:in find_library'
from extconf.rb:77:in `

'

To see why this extension failed to compile, please check the mkmf.log which can be found here:

/home/joe/.rvm/gems/ruby-2.7.1/extensions/x86_64-linux/2.7.0/mysql2-0.4.4/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /home/joe/.rvm/gems/ruby-2.7.1/gems/mysql2-0.4.4 for inspection.
Results logged to /home/joe/.rvm/gems/ruby-2.7.1/extensions/x86_64-linux/2.7.0/mysql2-0.4.4/gem_make.out

An error occurred while installing mysql2 (0.4.4), and Bundler cannot continue.
Make sure that gem install mysql2 -v '0.4.4' --source 'https://rubygems.org/' succeeds before bundling.

In Gemfile:
mysql2

Step 9/17 : RUN bundle install
---> Running in f9c34a3d5890
You must use Bundler 2 or greater with this lockfile.
ERROR: Service 'sidekiq' failed to build: The command '/bin/sh -c bundle install' returned a non-zero code: 20