omeroweb docker - add local app

[will-moore]

See discussion at: #2134 (comment)

Nice to find a more user-friendly way to add local app to PYTHONPATH in docker omeroweb.

[jburel]

@will-moore Does the recent work for omero-figure (cf. ome/omero-figure#469) a good compromise we could promote?

[will-moore]

yes - good idea - would we tell users to create their own DockerFile, as in
https://github.com/ome/omero-figure/pull/469/files?

[jburel]

We could add a Dockerfile to our repos and improve the doc.
I will look into it.