Fix docker#76

If the user runs that command "from the source code folder", i.e.
`getting-started`, docker will bind-mount `getting-started` to the
container's `/app`, which leads to error
`error Couldn't find a package.json file in "/app"`. It needs to bind
`getting-started/app` instead.
pwd can be confusing, so just be explicit about the path and make the
user user substitute the path in the command themselves.

docs/tutorial/using-bind-mounts/index.md

@@ -36,27 +36,27 @@ So, let's do it!
 
 1. Make sure you don't have any previous `getting-started` containers running.
 
-1. Run the following command from the source code folder. We'll explain what's going on afterwards:
+1. In the following command, substitute `path/to/getting-started/app` to the respective path to `getting-started/app` on your system, and then run it. We'll explain what's going on afterwards:
 
     ```bash
     docker run -dp 3000:3000 \
-        -w /app -v "$(pwd):/app" \
+        -w /app -v "path/to/getting-started/app:/app" \
         node:12-alpine \
         sh -c "yarn install && yarn run dev"
     ```
 
-    If you are using PowerShell then use this command.
+    If you are using PowerShell then use this command (remember to substitute `path/to/getting-started/app`).
 
     ```powershell
     docker run -dp 3000:3000 `
-        -w /app -v "$(pwd):/app" `
+        -w /app -v "path/to/getting-started/app:/app" `
         node:12-alpine `
         sh -c "yarn install && yarn run dev"
     ```
 
     - `-dp 3000:3000` - same as before. Run in detached (background) mode and create a port mapping
     - `-w /app` - sets the container's present working directory where the command will run from
-    - `-v "$(pwd):/app"` - bind mount (link) the host's present working directory to the container's `/app` directory
+    - `-v "path/to/getting-started/app:/app"` - bind mount (link) the host's `getting-started/app` directory to the container's `/app` directory
     - `node:12-alpine` - the image to use. Note that this is the base image for our app from the Dockerfile
     - `sh -c "yarn install && yarn run dev"` - the command. We're starting a shell using `sh` (alpine doesn't have `bash`) and
       running `yarn install` to install _all_ dependencies and then running `yarn run dev`. If we look in the `package.json`,