Add dockerfile & docker instructions into README

[gumernus]

The docker installation would be better for more exotic systems that struggle with installing things like openpress.

[gumernus]

And the customization part is nowhere to be found too

[HyScript7]

I don't know how to say this, but you're kind of the one responsible for that as the project lead 💀
Since we're here, let me ask: The customization part should be back-end wise with like plugins & stuff or purely on the front end with branding & layout?

[gumernus]

Well that will depend on what do want to customize on the backend xd