(MAINT) allow specifying container user

Extracted out of PR with multiple features in
5c194e4
and added tests and doc.

README.md

@@ -322,6 +322,7 @@ USAGE: puppet docker <action> [--from STRING]
 [--[no-]inventory]
 [--hiera-config STRING]
 [--hiera-data STRING]
+[--image-user STRING]
 [--puppetfile STRING]
 [--image-name STRING]
 [--config-file STRING]
@@ -347,6 +348,8 @@ OPTIONS:
   --hiera-config STRING          - Hiera config file to use
   --hiera-data STRING            - Hieradata directory to use
   --image-name STRING            - The name of the resulting image
+  --image-user STRING            - Specify a user to be used to run the
+                                   container process
   --[no-]inventory               - Enable or disable the generation of an
                                    inventory file at /inventory.json
   --labels KEY=VALUE             - A set of labels to be applied to the

lib/puppet_x/puppetlabs/imagebuilder_face.rb

@@ -46,6 +46,10 @@ class ImageBuilder::Face < Puppet::Face
         summary 'The default entrypoint for the resulting image'
       end
 
+      option '--image-user STRING' do
+        summary 'Specify a user to be used to run the container process'
+      end
+
       option '--labels KEY=VALUE' do
         summary 'A set of labels to be applied to the resulting image'
       end

spec/support/examples/imagebuilder.rb

@@ -20,6 +20,9 @@
       it 'the original from value' do
         expect(context).to include(from: args[:from])
       end
+      it 'no user set' do
+        expect(context[:image_user]).to be_nil
+      end
       it 'the original image_name value' do
         expect(context).to include(image_name: args[:image_name])
       end
@@ -51,6 +54,20 @@
     end
   end
 
+  context 'with a user set' do
+    let(:user) { 'diane' }
+    let(:args) do
+      {
+        from: from,
+        image_name: image_name,
+        image_user: user,
+      }
+    end
+    it 'should by include some label-schema labels' do
+      expect(context[:image_user]).to eq(user)
+    end
+  end
+
   context 'with a single env value specified' do
     let(:args) do
       {

templates/Dockerfile.erb

@@ -1,6 +1,8 @@
 <% if from %>FROM <%= from %><% end %>
 <% if maintainer %>MAINTAINER <%= maintainer %><% end %>
 
+<% if image_user && !image_user.empty? %>USER root<% end %>
+
 <% if environment %>ENV <% environment.each do |name, value| %><%= name.upcase %>="<%= value %>" <% end %><% end %>
 
 <% if rocker %>MOUNT /opt/puppetlabs /etc/puppetlabs /root/.gem<% end %>
@@ -138,4 +140,6 @@ RUN <%= puppet_path %> module install puppetlabs-inventory && \
 <% if entrypoint && !entrypoint.empty? %>ENTRYPOINT <%= entrypoint.to_s %><% end %>
 <% if cmd && !cmd.empty? %>CMD <%= cmd.to_s %><% end %>
 
+<% if image_user && !image_user.empty? %>USER <%= image_user.to_s %><% end %>
+
 <% if rocker %>TAG <%= image_name %><% end %>