docs: Add an example for configuring a custom DockerContainer with exposed port and environment variale

[vemonet]

Current examples for running a DockerContainer are all using with DockerContainer (3 examples out of 3).

But this is not the right way when someone wants to add some custom configuration (expose a port, define an environment variable), which is quite a regular use-case.

Newcomers reading this docs might get confused and will need to go through the process of finding out how to do this properly outside of the docs (e.g. this guy here #709 )

So I have added a basic example, feel free to change the docker image/port for something else

[Yourgene]

Hi, I had the same "issue" as you few days ago, and I had to look into code to find out that DockerContainer class is much rich than what is displayed in docs.
Thanks for adding it!

If I had a suggestion, I'd also add the volume mapping example, just for the sake of it. Volume mapping is a basic use case too, as the ones you provided.
If needed, here's the line you could add:

.with_volume_mapping('/path/to/host/dir/', "path/to/container/dir")