Quote volume mapping and use pwd

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

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

@@ -40,7 +40,7 @@ So, let's do it!
 
     ```bash
     docker run -dp 3000:3000 \
-        -w /app -v ${PWD}:/app \
+        -w /app -v "$(pwd):/app" \
         node:12-alpine \
         sh -c "yarn install && yarn run dev"
     ```
@@ -49,14 +49,14 @@ So, let's do it!
 
     ```powershell
     docker run -dp 3000:3000 `
-        -w /app -v ${PWD}:/app `
+        -w /app -v "$(pwd):/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 "working directory" or the current directory that the command will run from
-    - `-v ${PWD}:/app` - bind mount the current directory from the host in the container into the `/app` directory
+    - `-v "$(pwd):/app"` - bind mount the current directory from the host in the container into the `/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`,