Fixes #980 Adds Plugins to Persistent volumes in Dockerfile

[Nixis198]

No description provided.

[Difegue]

I wonder how this will work with updates -- If you install a new version of the server that updates the bundled plugins, will they overwrite the ones in an existing volume correctly?
That's the main reason I always left the plugin directory nonexposed, but I've never tested this stuff in detail..

[siliconfeces]

What about scanning for plugins recursively in the path? In that case, one can split it up fairly easily without causing too much issues with existing installs. Leave the bundled plugins in the base Plugins directory and store uploaded plugins in Plugins/Uploaded. The latter can be exposed as a docker volume/path if you want persistent uploaded plugins.

It will also be possible to install sets of plugins through additional mounts like Plugins/MyPlugins.

[Difegue]

The issue with that is that Perl uses the folder structure to do namespace lookups, so uploaded plugins would need a new namespace.. Although thinking on it there's no reason we shouldn't be able to bodge this with symlinks in the container.

Edit: although after doing some searching I might actually be wrong on that point? It's worth a try at least

[Difegue]

I've hijacked the PR to add a Sideloaded folder that Plugin uploads will now land into.
Seems like there's no issue having a bunch of differently namespaced files in that random folder, so it should now carry over between Docker installs just fine.