Remove images of the same name before pulling a new one

Or you'll end up with untagged dupes.

test/cli/build_test.rb

@@ -0,0 +1,15 @@
+require_relative "cli_test_case"
+
+class CliBuildTest < CliTestCase
+  test "pull" do
+    run_command("pull").tap do |output|
+      assert_match /docker image rm --force dhh\/app:999/, output
+      assert_match /docker pull dhh\/app:999/, output
+    end
+  end
+
+  private
+    def run_command(*command)
+      stdouted { Mrsk::Cli::Build.start([*command, "-c", "test/fixtures/deploy_with_accessories.yml"]) }
+    end
+end