Revert "Docker: Ignore ARGs without default values"

This reverts commit 27811af.

docker/lib/dependabot/docker/file_parser.rb

@@ -43,8 +43,6 @@ def parse
           dockerfile.content.each_line do |line|
             if ARG.match(line)
               key_value = line.delete_prefix("ARG ").split("=")
-              next if key_value.count != 2 # The ARG has no default value that we can set
-
               args[key_value[0]] = key_value[1].delete_suffix("\n")
               next
             end

docker/spec/dependabot/docker/file_parser_spec.rb

@@ -135,25 +135,19 @@
           expect(dependency.name).to eq("docker")
         end
       end
+    end
 
-      describe "with curls" do
-        let(:dockerfile_fixture_name) { "arg_from_curls" }
+    context "arg from" do
+      let(:dockerfile_fixture_name) { "arg_from_curls" }
 
+      describe "with curls" do
         subject(:dependency) { dependencies.first }
 
         it "can solve the dependency" do
           expect(dependency).to be_a(Dependabot::Dependency)
           expect(dependency.name).to eq("docker")
         end
       end
-
-      describe "without a default value" do
-        let(:dockerfile_fixture_name) { "arg_from_no_default" }
-
-        it "can't solve the dependency" do
-          expect(dependencies).to be_empty
-        end
-      end
     end
 
     context "with a non-numeric version" do