Perhaps add docker build to example_app docs

[jesperlandmer]

When running

docker run -p 5000:5000 example --name example

You'll receive an error message saying that

... pull access denied for example

Probably because "example" is not a public image.
You should perhaps add that you have to build the image specifying the name, e.g.:
docker build . -t example

[jesperlandmer]

Okay, so just found Toy_app repo, so this repo may be inactive