Expose plugins.local and themes.local via Docker volume

[Skyr]

Allow admins to install plugins and themes in a persistent manner (urgently required if you're automatically updating your Docker volumes via watchtower or similar) - mentioned in #9 :-)
Exposes two volumes; install and update process modified to keep original files from ttrss git repo to avoid conflicts during update.

[CorePoint]

Hey,
is there a reason this isn't merged yet? Would really like to add my own theme.

Greetings

[x86dev]

Seems like a very useful feature to me. However, I didn't find time to test this yet -- so the exposed volumes are purely optional, right? The patch looks sane to me ...

[Skyr]

Yes. afaics ttrss will search plugins both in the plugins and the plugins.local directory (same goes for themes). So this won't interfere with your distributed plugins.
The volumes are optional; without a volume mapping, Docker should create unnamed volumes without any content (which can be deleted when disposing the container).

[psyciknz]

I wouldn't mind this being merged, I have need of the options-per-feed plugin and it'd be handy to expose the plugins volume.