Add docs for building images with apps

[priyashpatil]

There must be docs for building images for a app. eg. Laravel. Instead of mounting app over the volume.

[jaydrogers]

See this for an example for now, but I will expand more to make this clearer in the docs:

https://serversideup.net/open-source/docker-php/docs/guide/migrating-from-v2-to-v3#cicd