(maint) Switch to ubuntu agent for network stability

When testing with the `puppet/puppet-agent-alpine` image on windows
systems with LCOW we had intermittent failures in DNS resolution that
occurred fairly regularly. It seems to be specifically interaction
between the base alpine (3.8 and 3.9) images with windows/LCOW.

Two issues related to this issue are
moby/libnetwork#2371 and
microsoft/opengcs#303

gem/lib/pupperware/spec_helper.rb

@@ -237,11 +237,19 @@ def clean_certificate(agent_name)
   # Puppet Agent Helpers
   ######################################################################
 
+  # When testing with the `puppet/puppet-agent-alpine` image on windows
+  # systems with LCOW we had intermittent failures in DNS resolution that
+  # occurred fairly regularly. It seems to be specifically interaction
+  # between the base alpine (3.8 and 3.9) images with windows/LCOW.
+  #
+  # Two issues related to this issue are
+  # https://github.com/docker/libnetwork/issues/2371 and
+  # https://github.com/Microsoft/opengcs/issues/303
   def run_agent(agent_name, network, server = get_container_hostname(get_service_container('puppet')), ca = get_container_hostname(get_service_container('puppet')))
     # setting up a Windows TTY is difficult, so we don't
     # allocating a TTY will show container pull output on Linux, but that's not good for tests
     STDOUT.puts("running agent #{agent_name} in network #{network} against #{server}")
-    result = run_command("docker run --rm --network #{network} --name #{agent_name} --hostname #{agent_name} puppet/puppet-agent-alpine agent --verbose --onetime --no-daemonize --summarize --server #{server} --ca_server #{ca}")
+    result = run_command("docker run --rm --network #{network} --name #{agent_name} --hostname #{agent_name} puppet/puppet-agent-ubuntu agent --verbose --onetime --no-daemonize --summarize --server #{server} --ca_server #{ca}")
     return result[:status].exitstatus
   end