Fix docker compose for WSL2 compliancy

[danielegiallonardo]

In a Windows+WSL2 environment, external volumes don't work.
In this PR a proposed solution with classic volumes

[peppelinux]

Thank you @danielegiallonardo

we should have some little changes on your very good proposal, these are:

1. Please merge the dev branch to get the last commits on the README
2. Please add an instruction to copy the example folder to another example_new or whatever, otherwise the changes we do on the dumps will affect the git tree.

Did you get it?
We should copy the example folder to another one to be ignored in .gitignore

once we have this I think that I'll abandon the external storage (+ thankfulness to you)

[danielegiallonardo]

I've merged the changes from upstream.
About the git tree, in order to execute the entire federation with an external RP, 4 files have been changed: the compose file (to include the section for the external RP) and the three dump files (to replace 127.0.0.1 with the hostnames).
I have a couple of suggestions about that:

• you could always use the hostnames (accordingly with the services included in the compose file) into your dumps, so it won't be necessary to modify them to launch the compose. This allows us just to make a "git pull" and a "docker-compose --build" on the django repo in order to have the new changes from the origin and have the data migrations run in a backward compatible way on the existing data, otherwise it would be necessary to re-copy the example folders each time.
• we could agree on a "modified compose" name to put in gitignore (to include the external RP section)