make own dummy, less confusing what is gem/engine and what is app

dummy/app/controllers/application_controller.rb

@@ -0,0 +1,2 @@
+class ApplicationController < ActionController::Base
+end

dummy/app/controllers/images_controller.rb

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+class ImagesController < ApplicationController
+  before_action :set_image, only: %i[ show edit update destroy ]
+
+  # GET /images
+  def index
+    @images = Image.all
+  end
+
+  # GET /images/1
+  def show
+  end
+
+  # GET /images/new
+  def new
+    @image = Image.new
+  end
+
+  # GET /images/1/edit
+  def edit
+  end
+
+  # POST /images
+  def create
+    @image = Image.new(image_params)
+
+    if @image.save
+      redirect_to @image, notice: "Image was successfully created."
+    else
+      render :new, status: :unprocessable_entity
+    end
+  end
+
+  # PATCH/PUT /images/1
+  def update
+    if @image.update(image_params)
+      redirect_to @image, notice: "Image was successfully updated."
+    else
+      render :edit, status: :unprocessable_entity
+    end
+  end
+
+  # DELETE /images/1
+  def destroy
+    @image.destroy
+    redirect_to images_url, notice: "Image was successfully destroyed."
+  end
+
+  private
+    # Use callbacks to share common setup or constraints between actions.
+    def set_image
+      @image = Image.find(params[:id])
+    end
+
+    # Only allow a list of trusted parameters through.
+    def image_params
+      params.fetch(:image, {})
+    end
+end