Update docker/entrypoint.sh to allow for sideloading of PIP packages (e.g. additional mopidy plugins)

[BasVanBoven]

On the Wiki, I would also add a sixth list item under ¨Getting-started > To install using Docker¨ to explain the new functionality:

6. If you want to sideload additional PIP packages to your Docker container, these can be specified using the PIP_PACKAGES environment variable, demarcated by a space between different packages

[jaedb]

Looks good! For users who wish to install additional packages, would this not re-install them every time the container was started? Naturally the delay to startup would be wholly dependent on the size and quantity of packages.
Is there a more efficient way we could tackle this by using volumes?

[BasVanBoven]

Hmm, that certainly is an interesting thought! However, for that to work I think we'd have to let pip store the custom package someplace else than the default location, make python read packages from there (perhaps through a path variable?) and also add a volume mount point. I do not know if it is possible to set that up. Also, it would introduce a lot of complexity for the adding of 1-2 packages use case.