Configurable app builds

[oliverbarnes]

On #53 (comment)_ @microstudi suggested:

Having different "default" images would be nice, but i guess just one would sufice if we adopt a strategy like Etherpad for instance:
https://github.com/ether/etherpad-lite/blob/develop/doc/docker.md

They have a single image but provide a script that makes super easy to build a new image with custom plugins in it:
![image](https://user-images.githubusercontent.com/1401520/98366290-fe7da480-2033-11eb-820b-7de1948ef5b8.png)

We could add a script to do the same with the Decidim modules, I'd even allow external plugins to be processed

I'm creating a separate issue for this as some of #53 is already being implemented in #50, which's already becoming a big fat PR.

And making the app build configurable is a discrete piece of work anyway.

[oliverbarnes]

Related: decidim/decidim#6918

[oliverbarnes]

Starting work on this. Focusing initially on configuring builds for feature branches of decidim repo, to facilitate staging deploys.