Error installing rubygems-update: rubygems-update requires Ruby version >= 3.0.0. #528

RUN gem update --system:
10.69 ERROR:  Error installing rubygems-update:
10.69   rubygems-update requires Ruby version >= 3.0.0.
10.69 ERROR:  While executing gem ... (NoMethodError)
10.69     undefined method `version' for nil:NilClass

make serve
cd certs && make test O=sets/test D=badssl.test
make[1]: Entering directory '/home/jozef/badssl.com/certs'
make[1]: Nothing to be done for 'test'.
make[1]: Leaving directory '/home/jozef/badssl.com/certs'
cd certs/sets && rm -rf current && cp -R test current
rm -rf common/certs/*.crt
cp certs/sets/current/gen/crt/ca-root.crt common/certs
cp certs/sets/current/gen/crt/ca-untrusted-root.crt common/certs
cp certs/sets/current/gen/crt/client.p12 common/certs/badssl.test-client.p12
cp certs/sets/current/gen/crt/client.pem common/certs/badssl.test-client.pem
docker build -t badssl .
[+] Building 12.0s (10/14)                                                                                                                                                                                            docker:default
 => [internal] load build definition from Dockerfile                                                                                                                                                                            0.0s
 => => transferring dockerfile: 703B                                                                                                                                                                                            0.0s
 => [internal] load metadata for docker.io/library/ubuntu:16.04                                                                                                                                                                 1.1s
 => [internal] load .dockerignore                                                                                                                                                                                               0.0s
 => => transferring context: 2B                                                                                                                                                                                                 0.0s
 => [ 1/10] FROM docker.io/library/ubuntu:16.04@sha256:1f1a2d56de1d604801a9671f301190704c25d604a416f59e03c04f5c6ffee0d6                                                                                                         0.0s
 => [internal] load build context                                                                                                                                                                                               0.1s
 => => transferring context: 1.36MB                                                                                                                                                                                             0.0s
 => CACHED [ 2/10] RUN apt-get update && apt-get install -y apt-transport-https                                                                                                                                                 0.0s
 => CACHED [ 3/10] RUN apt-get install -y software-properties-common                                                                                                                                                            0.0s
 => CACHED [ 4/10] RUN apt-add-repository ppa:brightbox/ruby-ng                                                                                                                                                                 0.0s
 => CACHED [ 5/10] RUN apt-get update && apt-get install -y     build-essential     git     libffi-dev     make     nginx     ruby2.4     ruby2.4-dev                                                                           0.0s
 => ERROR [ 6/10] RUN gem update --system                                                                                                                                                                                      10.8s
------
 > [ 6/10] RUN gem update --system:
10.69 ERROR:  Error installing rubygems-update:
10.69   rubygems-update requires Ruby version >= 3.0.0.
10.69 ERROR:  While executing gem ... (NoMethodError)
10.69     undefined method `version' for nil:NilClass
10.72 Updating rubygems-update
------
Dockerfile:16
--------------------
  14 |         ruby2.4 \
  15 |         ruby2.4-dev
  16 | >>> RUN gem update --system
  17 |     RUN gem install jekyll
  18 |
--------------------
ERROR: failed to solve: process "/bin/sh -c gem update --system" did not complete successfully: exit code: 1
make: *** [Makefile:99: docker-build] Error 1