Docker build failure due to asciidoctor-pdf-cjk

When attempting to build a local copy of the openapi files, I ran the .jenkins.sh script and received the following error:

Step 12/24 : RUN gem install asciidoctor-pdf-cjk
 ---> Running in ce3982ab7ed8
ERROR:  Error installing asciidoctor-pdf-cjk:
	The last version of public_suffix (< 6.0, >= 2.0.2) to support your Ruby & RubyGems was 4.0.7. Try installing it with `gem install public_suffix -v 4.0.7` and then running the current command again
	public_suffix requires Ruby version >= 2.6. The current ruby version is 2.5.0.

This patch fixes the problem:

diff --git a/docker/Dockerfile b/docker/Dockerfile
index b2bc215..38f0e96 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -13,6 +13,8 @@ RUN apk add openjdk8
 RUN apk add ca-certificates wget && update-ca-certificates 
 RUN apk add openssl
 RUN gem install rdoc -v 6.3.3 || gem install rdoc -v 6.3.3
+RUN gem install public_suffix -v 4.0.7
+RUN gem install css_parser -v 1.12.00
 RUN gem install asciidoctor-pdf-cjk
 RUN wget https://forge.etsi.org/swagger2markup-cli-1.3.2.jar
 RUN npm config set proxy $http_proxy