Don't install Jazzy on Xenial (#1597)

Motivation:

Ruby on Xenial is too old for the cocoapods downloader which jazzy uses.

Modifications:

Change Dockerfile to not install Jazzy on xenial.

Result:

Dockerfile will build again, but bionic or later is required for docs.

docker/Dockerfile

@@ -18,7 +18,8 @@ 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
-RUN gem install jazzy --no-ri --no-rdoc
+# jazzy no longer works on xenial as ruby is too old.
+RUN if [ "${ubuntu_version}" != "xenial" ] ; then  gem install jazzy --no-ri --no-rdoc ; fi
 
 # tools
 RUN mkdir -p $HOME/.tools

docker/docker-compose.1604.53.yaml

@@ -7,6 +7,7 @@ services:
     build:
       args:
         base_image: "swiftlang/swift:nightly-5.3-xenial"
+        ubuntu_version: "xenial"
 
   unit-tests:
     image: swift-nio:16.04-5.3