jgem is unusable

On a fresh install of Ubuntu 18.04.1 and 18.10, jgem can't install anything:

root@9adf8e6f577f:/# apt install openjdk-8-jre jruby
root@9adf8e6f577f:/# jgem install bundler
/usr/lib/ruby/vendor_ruby/psych.rb:6: warning: already initialized constant SNAKEYAML_VERSION
/usr/lib/ruby/vendor_ruby/psych.rb:6: warning: already initialized constant ANY
/usr/lib/ruby/vendor_ruby/psych.rb:6: warning: already initialized constant UTF8
/usr/lib/ruby/vendor_ruby/psych.rb:6: warning: already initialized constant UTF16LE
/usr/lib/ruby/vendor_ruby/psych.rb:6: warning: already initialized constant UTF16BE
/usr/share/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:59:in `require':
It seems your ruby installation is missing psych (for YAML output).
To eliminate this warning, please install libyaml and reinstall your ruby.
LoadError: load error: psych -- java.lang.NoClassDefFoundError: Could not initialize class org.jruby.ext.psych.PsychEmitter
           require at org/jruby/RubyKernel.java:955
           require at /usr/share/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:59
            <main> at /usr/share/jruby/lib/ruby/stdlib/yaml.rb:6
           require at org/jruby/RubyKernel.java:955
           require at /usr/share/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:59
            (root) at /usr/share/jruby/lib/ruby/stdlib/rubygems.rb:1
         load_yaml at /usr/share/jruby/lib/ruby/stdlib/rubygems.rb:716
         load_file at /usr/share/jruby/lib/ruby/stdlib/rubygems/config_file.rb:349
        initialize at /usr/share/jruby/lib/ruby/stdlib/rubygems/config_file.rb:202
  do_configuration at /usr/share/jruby/lib/ruby/stdlib/rubygems/gem_runner.rb:79
            <main> at /usr/bin/jgem:21

The problem doesn't seem to depend on the package being installed. I tried trollop, bundler and a local gem and got the same error for all.

At least the following yaml-related packages were installed as dependencies:

ruby-psych/bionic,now 2.2.4-6build4 amd64 [installed,automatic]
libyaml-snake-java/bionic,now 1.20-1 all [installed,automatic]
libyaml-0-2/bionic,now 0.1.7-2ubuntu3 amd64 [installed,automatic]

root@9adf8e6f577f:/# apt-cache policy jruby
jruby:
  Installed: 9.1.13.0-1
  Candidate: 9.1.13.0-1
  Version table:
 *** 9.1.13.0-1 500
        500 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status