Fix #76

[yuuyins]

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.
So, add /app after pwd for binding it instead, and be explicit to
the user to be on the source code's root directory getting-started
before running the command.

[StefanScherer]

Thanks. Ok, this is a thing we should definitely improve in the docs. But I prefer to keep $(pwd) in it to make it work when users want to copy-paste the command. I'd like to make the cd app more prominent, maybe as an example before docker run. I see that Run the following command from the source code folder. may not be obvious enough. WDYT?

[yuuyins]

@StefanScherer i think for didactic purposes, it would be appropriate to explicitly show in the command that we are binding app from the host. So I've added /app after pwd

-        -w /app -v "$(pwd):/app" \
+        -w /app -v "$(pwd)/app:/app" \

and made it explicit to the reader to be on the source's root directory. WDYT?

[yuuyins]

@StefanScherer i created another pr with the example you suggested #243. feel free to pick either of the prs and close either.

[StefanScherer]

Closing this in favor of your other PR.