Clarify how to create Dockerfile and how to build

Signed-off-by: Stefan Scherer <stefan.scherer@docker.com>

docs/tutorial/our-application/index.md

@@ -11,7 +11,6 @@ think about how it will work for a large team, multiple developers, etc.
 ![Todo List Manager Screenshot](todo-list-sample.png){: style="width:50%;" }
 { .text-center }
 
-
 ## Getting our App
 
 Before we can run the application, we need to get the application source code onto 
@@ -35,7 +34,7 @@ Dockerfile is simply a text-based script of instructions that is used to
 create a container image. If you've created Dockerfiles before, you might
 see a few flaws in the Dockerfile below. But, don't worry! We'll go over them.
 
-1. Create a file named Dockerfile with the following contents.
+1. Create a file named `Dockerfile` in the same folder as the file `package.json` with the following contents.
 
     ```dockerfile
     FROM node:12-alpine
@@ -45,7 +44,9 @@ see a few flaws in the Dockerfile below. But, don't worry! We'll go over them.
     CMD ["node", "/app/src/index.js"]
     ```
 
-1. Build the container image using the `docker build` command.
+    Please check that the file `Dockerfile` has no file extension like `.txt`. Some editors may append this file extension automatically and this would result in an error in the next step.
+
+1. If you haven't already done so, open a terminal and go to the `app` directory with the `Dockerfile`. Now build the container image using the `docker build` command.
 
     ```bash
     docker build -t getting-started .
@@ -64,6 +65,7 @@ see a few flaws in the Dockerfile below. But, don't worry! We'll go over them.
     for the final image. Since we named the image `getting-started`, we can refer to that
     image when we run a container.
 
+    The `.` at the end of the `docker build` command tells that Docker should look for the `Dockerfile` in the current directory.
 
 ## Starting an App Container