fix ruby version too old error

Author: yim-lee

docker/Dockerfile

@@ -19,8 +19,8 @@ RUN apt-get update && apt-get install -y lsof dnsutils netcat-openbsd net-tools
 # ruby and jazzy for docs generation
 RUN apt-get update && apt-get install -y ruby ruby-dev libsqlite3-dev build-essential
 # jazzy no longer works on older version of ubuntu as ruby is too old.
-RUN if [ "${ubuntu_version}" = "focal" ] ; then echo "gem: --no-document" > ~/.gemrc ; fi
-RUN if [ "${ubuntu_version}" = "focal" ] ; then gem install jazzy ; fi
+RUN if [ "${ubuntu_version}" = "jammy" ] ; then echo "gem: --no-document" > ~/.gemrc ; fi
+RUN if [ "${ubuntu_version}" = "jammy" ] ; then gem install jazzy ; fi
 
 # tools
 RUN mkdir -p $HOME/.tools